what gives you the impression its blown?
A general rule off the thumb is push the centre of the cone in (you MUST push in straight! IF there is resistance, and a scraping sound you have blown the voicecoils. Make sure you push it in straight, if you push in at an angle, you will get this scraping noise even on a brand new sub.
This could have been caused by sending a clipped signal to the sub/too much power/gains not set right etc etc.
That is just a rule however, dont take it as gospel, I've blow nsubs and they have felt fine, on the flipside ive had perfectly good working subs scrape when pushed in, but on the whole, scraping = fucked.