Yarn and Fabric related

These are my first ever proper granny squares ever!!!

As most of my fellow forum members know, I am absolutely not into knitting or crocheting as its something I have tried many times and never got on with. However I love wool for peg loom weaving. Well yesterday was the day I wanted to have another go at the craft of crochet and try to finally conquer it.   I did it!!! My first squares and I'm so chuffed.  I think I might now be hooked (excuse the pun).  Not sure what I’m going to turn these into. For the moment I’m having a lot of fun just making squares in lovely bright colours.  I do have a question for the experts below ....

How do you know what hook size to use when your ball of wool has previously been used and has no label?  I have a lovely stash of wool in gorgeous bright colours that I would like to use up, but none of them have labels.  One of the set of balls of wool that I used for these squares had a label that said "size 4" so that is what I have been using for these.