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?