There are several ways to approach this problem, depending on how much time and gas you have. The first, and most obvious, route is a scanner with tone search capabilities. Also, it often helps to establish a basic "perimeter" around the signal by driving certain routes and checking where the signal gets weaker or stronger. Eventually you should be able to discern whether the frequency is coming from a high-powered repeater on a mountain or tall building (obvious); half-duplex base station with simplex mobiles/portables, or 100% low-power simplex.
Sometimes low-power simplex can be the easiest to pin down, because the signal will only be strong when you get real close to the actual radio users. Even with simplex however, you might have a few radios with powerful base stations or mobiles situated some distance away from most of the other radio users.