How can I tell if a tire contains slime? I recently had an inner tube on my mountain bike changed,  but it was my wife that actually took the bike in to the shop for service.  When it came back I noticed the stem has a blue cap rather than the typical black cap. I have seen slime inner tubes that have a green cap, so it got me to wondering if there was something different about the tire. Although I hadn't asked for slime and she didn't remember anything being said about it, it's possible they could have just thrown it in as a freebie.  (This sort of thing has happened before where they quote one price over the phone and then the actual price winds up being less than what they quoted.) I have also noticed that at one point the tire went flat uncharacteristically fast (within a couple days of non-use compared to normal losses of a maximum of 1/3 pressure over a week), but after I pumped it back up it seemed to hold its pressure fairly well over a longer time period, which suggests to me that either there was a tiny leak which somehow sealed itself when the tube was re-pressurized, or slime was involved.

Aside from actually taking off the tire and taking out the tube and shaking it, how can I tell if a given tire has slime in it?
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.