How can you tell if the plugs need to be replaced? I have no idea how many miles are on them do to me having the car for 1 month or so.

 

If the little part in the middle looks all messed up then change them. If the car has trouble starting or bad mileage replace them. If they are black and cruddy change them. If you take some pics we can probably give you a good guess at if they need to be replaced or not.