I have a couple questions about car audio and wiring?

My first question is how do I know how many ohms my subs are, I bought them from a pawn shop type of place for a really good price because they knew nothing about them and they were taking up to much space. I know you can use a multi meter but I don't have one of those and I live in a rural area so audio shops don't really exist, I have 4 15" Memphis prs in a big sealed box, I checked the magnets earlier to see if it said how many ohms the sub was but itdoesnt so how else can I find out? Second question is I noticed that two subs are using the same terminal for each pair so 4 subs and only two terminals, what would be best a mono or two channel amp? Each sub has an rms of 250 watts so I know 1000 watt rms is what I need