Regarding car coolant: is it a big deal to mix (add new on top of old), since I don't know what brand/type of coolant was last put in the car?

I just noticed I’m seriously low on coolant and I would like to rectify this situation ASAP. I was reading my car manual and it says to not mix coolant types. I don’t know what kind of coolant is in there now, but really, it’s very low, so there isn’t too much of it.

I can’t figure out the diagram in the manual that shows where the drain plug is (yes, lame), so I’m tempted to just top it off with new coolant.

People who are mechanically-inclined: how serious would this be? Really bad idea? Moderately bad idea? No biggie at all?