If you've ever bought a house, you probably had it inspected beforehand. One thing home inspectors typically do is check all the receptacles (plugs, outlets) to make certain they're properly wired. You can do this yourself if you live in a rental or want to make certain the plugs in your residence are wired right. You should do this - an improperly-wired receptacle is a safety hazard and can also damage modern electronics.
Fortunately, a receptacle tester like the Gardner-Bender GRT-3500 only costs a few dollars and it's super-easy to use. You just stick it in the outlet as if you were plugging in a lamp, and analyze the circuit by comparing the three lights with the chart printed on the body. The most likely problem is reverse polarity, which means that the black and white wires coming into the outlet are reversed - pretty easy to fix if you have a screwdriver and a little patience (and know where to turn off the circuit breaker for that plug).
I've had to rewire a couple dozen plugs over the years at different houses, since so many DIY electricians can't seem to figure out which wire is which. Heck, the house I'm in right now was wired by a homeowner who installed half the light switches upside-down - I'm not real certain I can trust anything he did!
Since reverse polarity is a safety hazard, not to mention that reverse polarity can ruin expensive electronics, shelling out a few dollars on this handy little tool is a no-brainer. Get you one!