Christy,
A lot of the old spools of thread were marked with the thread size/thickness. Many if not most I've seen for sale now are not marked. For some of the current Coat & Clarks thread I've had to go to their web site to find out and that was a job.
One thing I do know is the C&C Machine Quilting thread is 30Wt or Size 50 ( same thing only different method of measuring ). Others have no info I can find.
Joe