If you are running the ACE Mod, you should never join a server that isn't running ACE.

ACE mod's design requires everyone connected to a server to use ACE, and it's highly recommended that everyone runs the same, latest version.

Many ACE servers have "ACE" in the server name, but you can find a list of dedicated servers running ACE provided by the Six Updater Portal.

For a full list of servers incl non-dedicated, check out Six Updater Client software.

(In Six Updater, Mods datagrid, right click @ACE and select "Find servers with Mod", etc)

 Also see Running an ACE Server.