When answering questions, I often find that I want to double-check that my answer is in-line with the most current version of the rules. Though I can often recognise that I need to check the errata when drawing on a book that I'm very familiar with, when an answer calls for drawing from many sources I usually won't have all that knowledge at my fingertips. What is the easiest way to know that a book has errata and where to find it?
Use the Wayback Machine
The Wizards of the Coast website used to have an archive page of downloads for all the 3/3.5e errata, which would have been found at this link while it still existed. Unfortunately, at some point in WotC's cycle of website redesigns it went away and they no longer seem to be hosting the files anywhere.
Luckily this is not a problem for us because the Wayback Machine has snapshots of the old site which we can use instead. The most recent working snapshot it has of that page is here, from November 2020.
To clarify: If the book is listed, then it has errata. Most of the file names also include dates.
The best place to start looking for errata is the archived errata page on Wizards.com. They're sorted by version (3.5/3.0), then by book. I don't know of any case where there's an official errata that isn't listed there. The Complete Champion errata is listed there, for example.
In addition, some 3.0 books have a conversion guide to update them to 3.5. These don't say they're errata, but they're also the only non-homebrew conversions from 3.0 to 3.5, so for something like the Monster Manual II they're essentially the 3.5 version.
Official Things That Are Not Errata
Then there's things that aren't errata but still put out by Wizards, like the FAQ (available over here). I only mention it to be able to mention that it's not errata. The same goes with the "rules of the game" articles that were published on their website.
These things can be useful and I've used them before when something particularly weird needed sorting out, bu they're not errata and don't override the core rules/errata. They do sometimes explain things better, and if they do change a rule, any DM is free to use the other version in their game if they want to.