leaky roof

I live in DFW area. I just noticed that an area of my ceiling has warter mark which indicates leaky roof. from what I can tell the leak must be very small because it rained all day today and I haven't seen it wet yet. But that part is not accessible via attic. so I don't know how to find leak.
I am calling a contractor to come over. but I'm curious how they detect leaks like this one to get myself educated so that I won't get ripped off.

another question is: what kind of shingles are considered as good quality in case I need to replace entire roof? My house is just an average house. so I'm not looking for super expensive shingles.

thanks for any tips

 

I might not even be from the tiles it could also be due to condensation in the attic.
A problem with some contractors is (since I have the same problem at the moment but in a much much worse scale) is that for projects like waterproofing they usually just come over, count how many m2 there are to put a waterproofing system on and just give you a quote and leave.
If you can find somebody who will actually locate the specific source of the problem and offer you a solution that fits you that would be probably better instead of having somebody fix the whole roof and maybe have the same problem again.
Maybe you can also try a few things your self first in order to fix it. Like trying to locate the source of the leak and maybe replace a few tiles your self.