I'm playing on OOO and not sure if MMM is enables or not, what's an easy way to find out?

Also, how do I enable OBSE so thieves arsenal works, I'm pretty sure I don't have it even though it's enabled under data files.

Go to the imperial bridge connecting weye to the IC. There is a barrel there with MMM hunting and crafting stuff. If you installed that plugin of MMM, you should see that barrel.

I am sure there are other ways to determine this (check Wrye Bash and see if MMM is activated etc.), but if you are not into using technical tools that much, just do that check in game.