swaping speakers in an ampeg 410

it seems like ive blown a speaker in my ampeg 410 cab. im getting a low fuzz sound and the cab wont handle very much from my amp. i feel certain a speaker has give up the ghost... but now i have the grill off and i cant figure out which speaker it is. its hard to tell for sure which speaker it is. is there any special ways of knowing, or can you only find out buy ear? anyone with any ideas or tips for me? thanks alot.