How to know if subwoofer is letting out to much air?

i mean i no , how u no if its letin out alot , but how wud u no wat sub will sound good , with alot of air rushing out or a little gap,, coz my cuz had gave me a kicker comp,(r dea gud) and i he made da box 4 it , but da air gap was da size of the hight of box , so wen i tryed it ,,, wen u set in car u can can kinda fell but her da bass more , n wen u lok at it , it dont , come , in n out dat much , so i was thinking da there is extra air coming out , shud i cover half da gap