I have a 95 XLT, this is my first sled so I don't know these motors all that well.
I'm getting coolant in the PTO and Mag cylinders, and they are not firing correctly. If I pull the drain plugs for each cylinder, nothing is really coming out, but I know it's getting into the cylinders because there is the milky mix in the pipes. Does this sound like a head gasket, base gasket, or water pump issue? I'm leaning towards head gasket, but need some second opinions.

Not the water pump, possibly a base or head gasket.

Rip it apart & replace both jug and head gasket...simple to do & cheap enough fix for a future headache. Inspect pistons while your at it...

Once you get it apart is there a way to tell if the gasket has been leaking, or do you just replace it?

Sometimes...but it depends on how bad the seal is/was. From the problem you stated, it probably is 1 or the other; or both leaking. There is normally a little bit of seal on both top & bottom of the head when you pull them off, and if it was leaking, there will be a little gap from where the seal had gone bad- probably real small & hard to tell, but you can sometimes see it. Or the seal will come off the head/jug real nice in one place and the steel will be wet from the anti-freeze in that place.

Easiest thing to do is replace both like I said before. Make sure to clean both surfaces real good & make sure their dry before re-assembling the parts.
-Good luck-

Turns out it was the o-rings on the SLP heads that were leaking. New rings and a little gasket sealant did the trick. Thanks.

thats nice to kno thoe if that ever happens to mine...how are the trails up there in jackman