How can you tell if your telephone is being tapped?

Now what exactly do you mean by "telephone"? If you're talking about a landline, you might hear a slight buzz or static in your phone if it's being tapped, but even that is very unlikely if very sophisticated methods have been used. E.g. if someone uses induction coil around your TP line and amplifies that signal to hear your conversation, you won't ever find out about it. If someone has simply tweaked the wire, attached another pair of wires with your existing line and is using a regular telephone to spy on you, you can probably find out because the signal quality deteriorates and you might hear some static/buzz.

Now if you're talking about cellphones, I don't think it's possible to know if it's being tapped or not. The most commonly used equipment to "tap" a cellphone is an IMSI catcher and trust me when I say that it's not something you can build in your basement. It's a very sophisticated 'machine', and fits in a big vehicle, a van most probably. It uses man-in-the-middle attack to tap your phone. You can find more info about this thing by Googling it. Criminals, as you see in movies, rarely use such type of equipment. It's beyond the reach of an ordinary criminal. Secondly, if fed agencies want to listen to your call, they can simply do that by asking your operator to record all your calls and they can get it anytime they want. Don't get paranoid about it. If it's the static in your cellphone that's bugging you, it's probably just interference.

By the way, there are many products on the market that let you spy on people. Just search for these terms on Google: "Tap a cellphone", "Tap a landline", "Tap a phone", "Wiretapping", and you'll find out a lot of info about this stuff. You can probably figure out if someone's using any of that fancy stuff to spy on you.

Good luck ;)

Note: All electronic communication can be tapped at all times. The US and our partners have a secret computer system that collects info and spies on everyone. You should watch this: http://www.youtube.com/watch?v=AH1lFyXSVrA&feature=channel_page