I find there's a lot of homebrew out there that has good presentation and seemingly good effort. But that doesn't make a homebrew good or bad. Balancing abilities thinking carefully about how it will fit into various worlds, making it flexible so the dm can work with it. Those are all basic things but without extensive knowledge of and experience in the game it's hard to figure out any of those. Basically I want to know an easy way to tell if a homebrew is good or bad.
There are several general metrics you can use to help evaluate homebrew quickly, though none are perfect:
Creating homebrew is an art, so if you find something you really like it's likely you'll similarly enjoy other work by the same author. Furthermore, the skills involved develop over time, so you're more likely to have less problems with newer material by your favorite artist than the first thing they ever tried to make. On the other hand, if you find something that doesn't work, you may want to approach future work from that source with a jaundiced eye, especially if your objections are stylistic rather than technical.
One of the best ways I've found to tell if homebrew is going to be a lot of work is to pay attention to the style of language used in feature descriptions it provides. People often echo the style of first-party material, but the degree to which they do so and the elements of style they pick up on vary. Imprecise and thoughtless language around mechanical bits is a good indication that the homebrew material is not going to go well in your game without a lot of work. Look out in particular for any rules that explicitly call out DM discretion or judgement calls without any advice or direction as to how said determinations are to be made-- this is a common shortcut used by inexperienced authors, especially with untested material, to avoid having to think about the consequences and/or details of a system.
Another important test that applies primarily to homebrew material you want to integrate into a fairly 'normal' campaign is a very rough check for balance and functionality.
For balance, look at the stuff the homebrew gives you. Is it vastly more than other stuff of the same resource cost? If so that's a problem. If not, then look at other things with similarish effects. Is the homebrew vastly more expensive than preexisting options? If so, that's a problem.
For functionality, the quick test I use depends on if it's longer material like a class or subclass or campaign, or shorter material like a race or feat or magic item.
For shorter material, look at the conditions you need to be in to use the homebrew material. Can those conditions actually arise? If not, or if they are extremely complicated to create and the material doesn't outline that, that's a problem. For example, a feat that (in part) gave you the ability to use an Action to increase your attack range with the Attack action for the rest of your turn would be usable only by fighters currently using their action surge. If said feat didn't acknowledge that anywhere, that would be a serious problem.
For longer material, look for contradictions or serious omissions. If the material contradicts itself or refers you to tables or statblocks that don't exist, that's a very bad sign. If you don't have time or energy to read through the whole thing, focus on the sections that are most relevant to your game (1st-5th level abilities for classes most likely, entry abilities and one-tier-up abilities for subclasses, the first hour's worth of material and the main baddies/locations/cool things for a campaign, etc).
Reputation of the finder
If you are auditing material a player has brought you, which is the time-limited situation I most often find myself in with regards to evaluating homebrew, consider most the reputation of the person bringing you the thing. Do you trust them to have evaluated it? Are they pretty knowledgeable about what's reasonable and not? How much do you trust their opinion of good v.s. bad homebrew? If you decide later that this thing they brought is no good and needs to go, how are they likely to react to that?
If you trust their judgement and willingness to work with you later if it's a problem, allowing anything is unlikely to be an issue. If you don't trust them to deal with the things being banned later well, and especially if they are a powergamer and especially especially if they are a powergamer and not good at dealing with/understanding rules you may well want to dissallow the homebrew.
Evaluating the "goodness" of homebrew is a tricky, subjective thing. My approach will not work for everyone, but it has served me well.
Take the large questions noted here. The more "yes"s you get, the more likely this homebrew thing is to be "good."
Is It Similar To Non-homebrew Things?
Stated in a different way: "Is the effectiveness of the homebrew thing (in the game) similar to non-homebrew things?" Make sure to consider quantity and magnitude of a thing's limits and strengths.
For example, a homebrewed weapon doing 100d20 in DnD 5e is wildly bad- look at the other weapons, where you're lucky to be doing 1d12!
If the homebrew thing has similar limits and strengths to non-homebrew things, both in quantity and magnitude, then it's good.
Does it Bring Interesting Choices?
This question covers the more tricky, often more creative homebrew options. Does this homebrew thing give players (or the DM/GM/Narrator/Voice/Etc.) more interesting play options?
What is an interesting choice? The ideal "interesting choice" has a clear impact on play (how players do things) and has no obvious superior choice. There may be other things that define 'interesting choice' in games, but these two criteria are a good starting point.
For example, sacrificing HP (in a DnD-like game) to cast a spell is interesting. The player needs to weigh their character's ability to stay alive with their ability to cast a spell, and that balancing is interesting.
Does It Fit the Fantasy?
This is super subjective and requires you to know who you play with and what they want to get out of play.
My example here: a group of spear-wielding, shield-bearing, anthropomorphic bunny Hoplites does not fit in your group's Serenity (sci-fi/western) RPG!
A Final Note
Sometimes good homebrew goes bad. If you, as a GM, are tempted to show favoritism to your homebrew thing- like your half-dwarf-half-elf-with-dragon-blood hybrid never dying in spite of the best efforts of your players and no narrative reason, you've taken potentially good homebrew and turned it bad. Don't do this.
Good homebrew stuff, when well implemented, serves to increase the fun of every person who is playing that RPG. It fits in, it is not forced in.
Homebrew is "good" when it is mechanically balanced, interesting, fits the fantasy, and is NOT preferred over non-homebrew.