Sudden loss of hearing, gradual loss of hearing, ears ringing after use, after using headphones normal sounds seem quieter than normal.
If you are only plugging them into your PC with your soundcard, it does not output enough power to damage them. The point is that you will likely go deaf from them before they are destroyed by a PC soundcard.
If you are using them with a receiver you will have to turn it up to the point it will damage your ears in short order.
The classic signs that you are putting too much power through a speaker include 'bottoming out' (where the coil strikes the magnet), smoke, or the smell of something burning.