What I've done was wait until dark and used a flashlight where I'd hold it 'parallel' to the roofline and look for a reflection. Sometimes, holding the light at almost any other 'angle' didn't work but you might have some luck. Of course, I did this when there was a heavy rain.
Also, you can look for drip spots on the floor or areas along the top-plate where water will run (if it's not dripping before it gets to that point.) Usually, the water is going to drip or run.
My best to ya and hope this helps.
Jay J -Moderator
PS: USe a silicone caulk if the leak is at a nail spot. If the leak is coming from a spot where 2 pieces of sheathing meet, you should remove at least 2 rows above the leak (as well as the row where the leak is) and put some tar paper down. Don't forget to caulk the nails where you've installed the new shingles just for insurance.