This is always an annoying problem. I once had JUST the ADC0 pin on my ATMega8 fry. I couldn't figure out what was wrong for so long. The chip was connecting correctly, I was able to upload code, and every other function seemed to work correctly. I spent literally days trying to figure out what was wrong. I eventually just replaced the chip and it worked fine.
lesson: No easy way to tell if a chip is partially fried.