I remember when you could always find the ply in yarn, but looking on the labels.
I like to use 2 ply baby yarn. How do I find this when some of these new yarns have no : 2 ply, 3 ply, etc. on the labels?
Ply as a system of weight is used mostly in Australia and sometimes the UK now. Here's an article that shows the comparisons. The 2 and 3 ply baby yarns are usually called fingering weight now, sock yarn might also be about the same.
There are so many different types of yarns now that ply is not quite as effective in describing yarn now, too. You can have two ply and have it be worsted or even thicker. Depends on how it's spun. http://www.purlbee.com/2-ply-4-ply-why-ply/
When asking questions ALWAYS post the name and a link for the pattern if you have it.