If you have rain water running behind your gutters then it is likely that you have clogged gutters.
Rain water running behind gutters.
Capillary action is a water activity in which water adheres to a surface and follow the surface around.
If the gutters are clogged it s possible that the water is overflowing the gutters and this is contributing to the leak.
Many of the solid gutter covers are design to take advantage of this water characteristic.
If the gutters look clear run a hose and fill the gutters with water.
Just as you had thought of doing.
The first cause is when the roofing material is installed many roofers will not extend the roofing material past the roof edging or drip edge.
The purpose of this product is to prevent water from running behind the gutter.
This product is an easy and inexpensive fix.
The exopsed part is not long enough about 1 long for the gutters to tuck under and can create problems with water running behind gutters.
Unfortunately this is a very common issue with rain gutters resulting in damage to the roof s edge as well as the structure s foundation.
If this is the problem a simple solution is to slip some trim coil or sheet metal up under the d edge and flash it over your gutter.
Water should run out the downspout and away from the house without filling the gutter.