Are there tests to determine if you need a root canal? If not, how do they know if you need one?

Before Christmas, I had an amalgam removed due to deterioration. There was decay underneath and it was drilled away and filled with compositve. Thereafter, I had slight pain only when eating crunchy foods. It did seem to get better or perhaps, I just got used to it. Now, they have removed it, put in a temporary filling, and are going to fill it again next week if I don't have any pain. If I do have pain, they are considering a root canal. Since the pain is minimal, are there other tests to determine if a root canal is necessary? Or, is pain the only factor? I hate to go that route if it's not necessary. But, if it is, I would like to have it taken care of now while I have dental insurance as well as a temporary filling. Can any dentists advise me about what the determining factors for a root canal are? And, do you recommend another composite or amalgam for my lower molar? Thanks so much for any advise! =)
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The tooth should be checked with dry ice or an electronic pulp tester. A painful response to either one would indicate the need for you to have a root canal. After the root canal, the tooth should be refilled with "either filling material" and then crowned to prevent fracture.
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