I use Ingress, Niantic's previous AR game. If I'm in an area where I don't know the nearby spawn points, you can find some by opening Ingress. In Ingress, many Pokemon spawns are at the same location as XM, which are small clusters of lights. PokeStops and gyms likewise are in Ingress as larger, more complex structures with tons of lights around them. If you're in an area with many of those, this trick will not work, as the gyms and PokeStops will obscure the XM. In these cases, I fall back to the triangulation method that GrizZ outlines in his answer.
I try to avoid all that hassle by being prepared before going out to hunt Pokemon. If you're technically inclined (and don't mind breaking the TOS, with the understanding that it could get you banned), you can setup scanning software to scan the area to identify spawn points and what's spawning. Alternatively, you can search online for local Pokemon Go communities, as they will often have spawn points mapped, or at least where nests are located (nests are where specific Pokemon spawn often). I use these so I don't have to depend on the Sightings feature to capture the Pokemon I'm seeking.