If you actually 'blow out' a speaker by exceeding its stress limit, the cone will separate from the voice coil glued to it, or the voice coil will fly so far off the pole piece that the wire tears off. Very easy to tell, just by listening. Blown speakers either don't respond at all, only give out fuzzy distorted sound at high peak levels, or partially blown will work but distort severely.. so you will know immediately if it is blown by putting the stereo on a normal to high volume level, balancing and fading everything to 0 (even) and listening to each speaker. Also, if they are woofers, just touch the cone and see if they are responding to bass properly. Blown woofers hardly move at all. Almost all speakers I have run into that blew the voice coil don't respond at all except for a nasty, fuzzy burpy sound on high current peaks like bass kicks. If you want to get technical, get a multimeter, probe the terminals with the system off and run through your impedance range (ohms) - a speaker that separated from the voice coil will read infinite impedance at all settings. A good speaker will read 1.0 at 16/8/4/2/1 Ohms, depending on the system. (usually 8)