How can someone tell if a cell phone is being tapped?

There usually will not be obvious signs of a trace. If your phone is warm when it shouldn't be, or has dramatically decreased battery life with no increase in usage, it might be bugged.

Also, if you have a GSM phone, and your phone causes speakers (e.g. from a stereo or computer) to buzz when you are not on a call, then it might be tapped.

Of course, there are other reasons that could cause any of the above situations.