How much it cost to replace Alternator is Nissan Maxima 2001 GLE Dealer told you that you have to replace the alternator How can you make sure that it is really needed to be replaced?

Depending on where you live, you MAY be able to find a local auto parts retailer that has the ability to test your alternator. They generally test the entire charging system and do not charge for the service since they're trying to get you to buy parts from them. You're right to not trust the dealer, their mechanics are paid partially by commission and tend to want to replace as many parts as they can.