Jon is right. The best ID guides are online. For Singer machines, I think the Sandman Singer ID Template is very good:
http://www.sandman-collectibles.com/...r-machines.htm
For non-Singers, I just Google the brand and model and look for reviews. That's how I first found Quilting Board as threads here often pop up. I have used my phone in thrift stores to look up machines I wasn't sure about. There's another site with machine reviews which often comes up and it's very helpful but I don't think I'm supposed to name it here.