Your best bet is a fairly good stud finder. I've found that the cheapest ones are just that, but, the mid-priced and up are generally fairly reliable. Use one to "find" the rafter and then try the "thump" test to verify. You should be able to hear the difference and then some judicious pushing on the ceiling should let you feel it.
Once you've found what you think is the rafter next to where you want to put the skylight, you'll need to take some measurements and go up on the roof to make sure that the area is clear of antenna, vents, etc. for an area slightly larger than the skylight. You'll need ~ 4" ( a WAG on my part, maybe the instructions in the skylight will give a more accurate measurement) all around the skylight to allow for flange and sealing of the joints.
Once you have all of that set, the really tricky part comes. This is where the patience and some small amount of luck comes into play. I would drill a series of ~1" holes along where you think the rafter lies (drill along the scrap side of the rafter), in a line extending not quite the distance of the opening (around 12" for a 14" opening?). Use those holes to poke with a bent coat hanger to verify that the rafter is along that line and then also use those holes looking for wires, ducts, etc. You would be amazed at where the manufacturers sometimes run wires, not just 12v, but, also 120v! Go carefully. Once you have the existing rafter line established, you'll need to make a square, truly square, template of the size opening you'll need. Again, refer to the skylight instructions. There may be an "offset" that you need from the rafter...your instructions will give you all the proper measurements. Once you have the proper sized template, and have it lined up with the rafter, tape it in place and mark the four corners. You'll then have to use a 1/4" drill or slightly larger, to drill up through the ceiling and the roof to mark the four corners on the roof. A 1/4" bit (or slightly larger, but, not much) will give you the clearance necessary for a jig saw blade to cut out the scrap. From that point, you'll need to cut blockers to fit between the ceiling and the roof for the 3 sides that aren't bordered by a rafter and glue them in place. The rest should be fairly straight forward. I would think the skylight with inside trim would go down followed by the external trim ring with lap sealant and screws. Last would be the inside trim ring with screws into the rafter and blocking that you put in.
Again, please refer to any instructions the skylight comes with. I'm just "ad libbing"; how I would do it if I had to do it sans instructions. Work carefully, keep it square and measure, not once, not twice, but, three times and make sure all three measurements are the same...you can make a hole bigger, but, you can't make one smaller and you can't say "Ah, phooey, I give up" once you've put holes in the roof!
Much luck and let us know how it comes out!