Q: Serpentine Belt broke How do i tell if its the Altenator

A lot depends on *why *the serpentine belt broke. If the belt broke after 120,000 miles simply due to age, that’s fine, but if it broke because one of the rotating elements - the tensioner, pulleys, alternator pulley, and so forth - has a seized or rough bearing, unless you fix THAT issue, the new belt will break as well. So, before you install a new belt, all of the rotating elements with bearings have to at least be checked for roughness and noise, although once any pulley bearing get past a certain mileage, they will often just be replaced anyway because they most decidedly do not last forever (what happens is the grease inside these pulley bearings oxidizes and dries out due to the high heat from the engine). As far as the alternator, once you re-install a new belt, it is easy to test for correct electrical output and you definitely should do that. If your car is starting and running only briefly you might have an issue with your battery (the battery can be load tested to rule that in or out as an issue) or a more involved engine management issue involving the fuel, ignition, and/or air induction system. Your best bet is to just schedule a diagnostic of your starting issues, during which a certified YourMechanic technician can tell you exactly what needs to be done to get your car fully operational again, in addition to helping you with any belt replacements that need to be performed on your vehicle.