all belt driven components will subject a belt to more or less torque, and the engine will subject torque to the system upon revving. that is, it's not as simple as "everything is the 'same', why do I hear squealing only sometimes?" this is because components only require power sometimes, and the engine only produces more power at high RPM.
for example, when stopped, and low idle RPM, and turning your steering wheel; this demands power from the steering pump that the engine isn't necessarily supplying readily at low RPM. mine makes a strange sound. it's fine at high RPM, or not trying to force the steering wheel when stopped. another example is that upon startup, the alternator is topping up the battery for about 5 seconds; when the alternator is forcing current to the battery, it puts a strain on the belt as it converts mechanical energy into electrical. when it's done charging, the current is cut, the alternator does not resist torque as it did previously, and it does not strain the belt.
now the engine will supply higher power upon revving, and all components will change again. it's all very unpredictable unless you know how it works.
what I've done in the past, is gotten someone else to startup the car, (rev if necessary), and press my ice scraper against various belts until the squealing stops. that is, forcing the belt inwards with an object is like adding a "belt tensioner" for a moment until you find the belt that's squealing. DO NOT FORCE THE BELT TOO MUCH. DO NOT HOLD THE OBJECT AGAINST THE BELT FOR TOO LONG. DO NOT USE A SHARP OBJECT. DO NOT USE YOUR HAND. WATCH OUT FOR THE BELT GRABBING THE OBJECT AND THROWING YOUR HAND INTO THE ENGINE OR SOMETHING NASTY. DO NOT PUT ANY OBJECT INSIDE THE BELT LOOP - ONLY APPLY PRESSURE FROM THE OUTSIDE, INWARDS.
be VERY careful when you're sticking things into belts.
now, if you're worried about your alternator, check to make sure all your dash lights aren't on (that means it's not working altogether). check your voltage meter; if it doesn't eventually rise to mid-point (12 ish volts), and the squealing continues, it's your alternator or alt belt. usually though, an alternator belt will only squeal at startup, and then stop when the battery is charged, and only squeal at lower RPM, not higher. check your alternator brushes if you're really worried. I got a friend's 240 alt brushes replaced and installed for $20.
Sim