I don't know about pushing on the sub, but you can get a piece of TP and hold one end and then run it over all of the seams to see if it flaps then you've got a leak.
Also you could just reseal the whole thing, but wait there's more, you could then rewire and recarpet then get new subs, wait what the hell am I saying just go to Walmart and buy one of those kicker boxes with the neons, now seriously folks, I'm just kidding, those boxes are trash.