I need a Roofing Repair Contractor

 Evidence of prior leaks is not typical and they are usually discovered by the inspector on what we call the "decking" of the roof.  The decking is the plywood layer of roofing to which the felt and shingles are attached. 

You (and your inspector) can visually inspect the decking from the attic where the underside of the plywood will be exposed.  Large water spots, dark areas, and rotted plywood will be evidence of a prior leak.  If you see visual evidence of a previous leak, I'd highly suggest having your agent do some investigating by asking the current owner if (a) they are aware of the leak; and, (b) the leak has been repaired.

1 thing that is not normal in roofing is missing sections of shingles.  I'm not talking about one or two shingles here and there.  I'm talking about large parts missing from the roof.  As a buyer, you will most likely not be climbing on the roof to inspect the shingles.  This is where your home inspector is vital.  Most home inspectors do one of two things to examine a roof - (1) they will physically climb onto the roof and "walk it" to inspect its condition.  Or, (2) they will position and climb ladders to where they can visually inspect the condition of the roof through binoculars. 

Personally, I would rather the inspector "walk" the roof.  I think it provides more of a comprehensive inspection.  But, visually inspecting through binoculars seems to be almost as good from my experience.