Also, depending on the company, if you read the descriptions carefully you will find some clues.
Of course they're not going to say "this yarn is scratchy as hell!"
But I look for phrases like "soft enough for next-to-the-skin wear" Or if they specifically say it's soft enough for items that will be born against the skin (like a hat), that helps too.
Most companies, from what I've seen so far, will not specifically say that it can be worn next to the skin if it's not pretty soft and non-itchy. But, YMMV since that's a very individual thing.
One time I bought some yarn and was disappointed at how scratchy it was. I thought from the vendor's description, it was supposed to be pretty soft. But I went and looked at the description again, and could see that they say it has some softness, but they really didn't say anything about it being super-soft, or OK for next to the skin, or anything like that.
So, those are some things I look for. Also, unless I trust the vendor and the description on something is pretty specific, I get advice online before I buy stuff.