A roof rarely fails all at once. It fails at the edges and the details, over years, and the stain on the bedroom ceiling is the last step in a process that started long before. If you know what to look for, you get to replace a roof on your schedule and your budget instead of during the first week of February.
From the ground, look at the lines. A ridge that dips, a plane that waves, or a section that looks darker or flatter than the rest usually means the decking underneath is wet or soft. Look at the granules too. If the gutters are full of what looks like coarse black sand, the shingles are shedding the layer that protects them from ultraviolet light, and once that is gone the asphalt goes quickly.
From the attic, go up on a bright day with the lights off and look at the underside of the deck. Daylight anywhere it should not be is obvious. So are dark staining around nails, rusted fasteners, and matted or damp insulation. While you are there, check whether the soffit vents are actually open. A lot of roofs in this area die early because someone insulated over the intake vents and the deck has been cooking and sweating ever since.
After a hard winter, pay attention to the eaves. Ice damming is what happens when heat escapes into the attic, melts snow at the ridge, and the water refreezes at the cold overhang and backs up under the shingles. It is the single most common source of leaks in older New Jersey houses, and it is fixable, but the fix is ventilation and ice-and-water shield at the eaves, not another layer of shingles over the problem.
One more thing worth knowing: layers. If your roof already has two layers of shingles, code requires a full tear-off next time, and that changes the price. Find out which you have before you start collecting quotes, so you are comparing the same job.