Engine identification is easy no matter what year: the 3.1/3100 will have either "3.1L Multi Port" or "3100 SFI" stamped into the top of the engine, the 3.8/3800 will just have a plain black intake manifold (some will have a dustcover that says 3800/Series II). I agree with dwalmop, the intake goes out like clockwork on these engines. I'm due for my own Lumina's gasket soon, I'd rather not wait but money's tighter than ever.
That and the sign that you have to keep adding coolant indicates its going SOMEWHERE. Coolant doesn't just disappear; its either getting burned, going to the oil, or going to the ground. If there's no external leak, it's almost certianly the intake gasket. I was quoted $750 for parts & labor from three shops for the job.
Its a fairly detailed process, but the $15 haynes/chilton manuals will tell you step by step how to do it. There's a lot of parts to disconnect and remember, and there's actually some "newbie" instructions on how to remove the heads (the intake has to come off for that, so it's good to that point) elsewhere on the forum.
It might very well be warped heads, but the probability of that is much MUCH lower than the gasket issue. Hopefully you caught it early enough to where nothing major was damaged by coolant (bearings get eaten up quick by this stuff).