The DMG shows you how to determine if something is, in the abstract, an appropriate challenge. From the SRD:
A monster’s challenge rating tells you how great a threat the monster is. An appropriately equipped and well-rested party of four adventurers should be able to defeat a monster that has a challenge rating equal to its level without suffering any deaths. For example, a party of four 3rd-level characters should find a monster with a challenge rating of 3 to be a worthy challenge, but not a deadly one.
Monsters that are significantly weaker than 1st- level characters have a challenge rating lower than 1. Monsters with a challenge rating of 0 are insignificant except in large numbers; those with no effective attacks are worth no experience points, while those that have attacks are worth 10 XP each."
The stock satyr is CR 1/2, which means your party of four or more level 1 adventurers should rock its world.
But Parties Differ
But this is a general guide - only you know your party and its current state. Will they have had a lot of fights by that point without resting and be already significantly banged up? Do they kinda suck because they're new and have struggled with other CR 1/2 foes? Does the monster have abilities that the whole group is super weak against for some reason? (In this case, if you just had a party of all wizards, the magic resistance of the satyr would be much more impactful.)
And Monsters Differ
The DM controls the difficulty of an encounter to a great degree by determining the creature's motivations and tactics. If it cleverly splits them up and uses ambush and focuses on weakest opponents first and fights till dead, then that's a hard encounter. If it spends every other action wiggling its furry butt at them to taunt them and runs away if wounded, then it's an easy encounter. An experienced DM can, on the fly, always lower the difficulty and can usually up the difficulty by a level or two just by how they play the monster. And losing a fight doesn't have to mean death... Maybe this satyr needs some wingmen to win that nymph's heart and the PCs fit the bill.