A few things to check to get a better assessment:

1. Do the profiles of the teeth on the middle chainring look different from the others. Often, worn chainrings have an asymetric shape, resembling shark fins. It's often quite apparent.

2. Measure your chain and check for stretch. Park Tool's site shows how to measure with a tape measure or good quality ruler. Or you can buy a chain gauge tool (yes, Park will sell you one).

If your chain is stretched beyond recommendation (I think 1/8" over a 12" span), then most likely your cassette is trashed. If you chain is within allowable spec, you might be able to get away with just a new middle chainring.