Hi, Terry. This depends a lot on individual situations. The first thing to do is check around for clues like any evidence of nail heads or holes or even a little evidence of a "smiley face" that a hammer has left. You might be able to draw some conclusions from that, even though the clues are away from the area where you want to mount the swing. Don't forget to look all around the soffits and eaves as this can tell you what's underneath. And keep in mind that you might be dealing with roof trusses which are sometimes on 24" centers rather that the 16" centers of a stud wall.
If you find no evidence at all, which really would be surprising, drill one hole and use something like coathanger wire to probe to the sides to find the joists. once you have the location of 2 of them, you can figure the pattern and have a good idea where all of the others are.