usually if there is color #, and amount of thread on the spool there is also a number...30, 40, 50...that is your thread weight. 50 being quite thin, 30 the heavier...my nephew came home with a $12 spool of 12 weight once...found out none of our machines would accept such a heavy thread...we all started paying a little more attention to what we were buying..,
i think it is Signature threads that has a pretty good tutorial about threads...