How can I tell if a tire contains slime?
To find out if there is sealant inside the tube, 


cut it open - kinda terminal for the tube
take it off the rim and deflate it then squish the tube with your hands.  It will feel more slippery than an empty tube
Hang the deflated tube up for a night and the sealant will pool at the bottom.  You should be able to feel it through the tube
If the valve core is removable, remove it and stick the end of a matchstick or toothpick through, see if it comes back damp.


Generally weight alone won't tell you, because there's only 20-50 grammes of sealant in a tube, when a tube weighs 5 times that.