Is there a way to find a griefer on my server? he has been dropping Origin bosses (playing Primal Fear) in peoples basses, killing their stuff.

Well normally you would have anti-cheat turned on your server, and well as battle eye, which prevents most of the griefers/hackers.

You would also be able to see them via the players tab.

Also, setting an admin password on the server, as well as requiring a password to join or whitelisting the server only is another layer of protection you can implement.

Finally, player tribe logs should be able to indicate who owns the dinos in question that are griefing, and you can then check those names versus a program to load player profiles to find out which steamID is doing so.

the problem is he is spawning a boss in that no one owns. was hoping there was a way i could see who spawned it

Ah, in this case the mod itself may have logging enabled. I would check the mod's documentation and see if it has anything robust like that.