My gaming group has finished all of the official MoM scenarios a couple of times, and have been looking for some good custom fan-made ones to try out. However, I'm not even sure where to start looking, or how to tell if a given scenario is any good. Where do I find good custom scenarios for the game, and/or how do I tell if a scenario is good?
I would advise that you check out the Mansions of Madness page on BoardGameGeek. Specifically, I would look in the Files section to find more custom scenarios. Although I'm not clear how your group would define "good", a reasonable way of evaluating the good and interesting scenarios is to see how many Thumbs Up each File has been given. A quick way to determine the most popular entries is to sort on the "Hot" rating of each file.
Please note that not all the Files listed are scenarios. Several files are player aids and reference sheets. Also the site supports multiple languages so some scenarios may exist that are written in a non-English language. A quick perusal of the Files shows several user created custom scenarios have fairly good ratings (20+ Thumbs Up).