If you are a member of Project Nebula, Dominion’s sister minecraft server, you can be giving the Dominion Staff role, and this will give you access to the #ban-list channel, in the Moderator section of their discord.
In the #ban-list channel, Nebula posts a record of each person that is banned from their server with their discord username, their in game name (IGN) and the reason.
It typically looks like:
Discord: Username#001 IGN: MCusername Reason: x-ray, flyhacking
click into the search box and type either:
in: ban-list Username#001 or
in: ban-list MCusername
And if the user was banned, it should come up as a search result. However Nebula does not include the discord ID with this information so its always possible a player might change their discord name. The best way to search is using the MC username as that is less likely to have been changed. (And can be tracked via nameMC if it has.)