Some of the things that I look for when someone thinks they need to replace their roof is first "How old is it?" most people replace their roof every 17.5 years on average, then I start looking at the shingles themselves, "Are they curling?" "Is there a good amount of fiber glass showing?" or "Bare asphalt?".

Attic inspection is always a good place to look as well, usually when I go into an attic I first lookup to see if vent holes have even been cut open for the roof vents, you would be surprised at the stuff I see, I look for blackened or moldy plywood and blockage of intake vents we don't do insulation but if it really needs it I have a specialty contractor that I refer that out to.

This is some water staining. Rake metal is a very important part of a roof system.

Ice & Water shield is also very important 

I alway recommend tearing off the old roof and inspecting for bad plywood, it is very important that we have a solid foundation for installing a new roof, did you know a "Lay over" may only last as little as... Lets not go there a complete tear off is always best it may seem to be a cheaper option however it will cost much more in the long run.