Answers

First have your roofer do a visual inspection for obvious areas where water could enter your home.  Then, do a simple water test.  Have one person look in the attic and one person in the home.  Have a roofer go on the roof with a running garden hose. Have the roofer start introducing water at the lowest parts of the roof and work up.  Have all 3 people on a cell phone conference line so that if water is seen coming into the attic or home the person on the roof can shut off the water. 

If you can get into your attic you should be able to see evidence of leaks. Look for staining and/or rotting wood at walls and penetrations.

One easy way is to simply go into the attic space and look around and see if there is any staining on the underside of the ply wood.  The most common areas are around pipe and chimney penetrations, in the valleys or where the roof terminates into a wall.  If your roof is walkable, and you feel comfortable enough to get up there, you can also use a garden hose and attempt spraying the roof from different angles to see if any water gets through.  Over all though, if you see any discoloration at all, then water is penetrating some way.

Roof to wall intersections and pipe collars are probably the two most common locations. What is the height and pitch of the roof? I don't suggest hitting the roof with a bunch of water unless you know how to spray it. Shooting water from the bottom up can facilitate a leak on a roof that is otherwise working properly.

An attic inspection is a good idea an if you can separate out the difference between bulk water and condensation stains, you should be able to put an eyeball on the leak.

Good luck and do not get on the roof unless you know how to walk it and have PPE.

WoW Home Solutions.

The BEST way would be infrared image testing. Only top-tier roofing companies will have them, so may be hard to locate in rural areas. However, the technology is getting older, and the cameras that were $1k, 2 years ago, are around $399.99. Its a solid investment, and requires none of the old, trial and error methods.