MCBuddies is a community on Reddit where you can find Minecraft friends and server to play on.
MCBuddies has a stickied post at the top of their page where players can post about people who have caused issues on their servers or while playing minecraft with them. At the time of this guide there are four posts. The oldest three are locked and fourth one is active.
Central Post for Troublesome Players 1 🔒
Central Post for Troublesome Players 2 🔒
Central Post for Troublesome Players 3 🔒
Central Post for Troublesome Players 4 🔑
Main Community: https://www.reddit.com/r/MinecraftBuddies/
If you check and the fourth one closed, try checking the front page of the community for the newest stickied post.
On each post players post information about bad players they have encountered through MCBuddies. This might include MC usernames and the reason they caused problems. Usually the Discord username cannot be included as per their rules.
You can search through each post to see if anyone listed is known to the server, or when you are doing a deeper background check on someone just to see if they are listed. But for basic background searches its probably not necessary to check this site.