Am I the only one who finds it frustrating that so many patterns nowadays are specifically for certain wools and not just for three ply/ Dk etc? There were some lovely wools in the John Lewis sale today but they were not specifically 3/4ply etc so I wouldn't know if I had any patterns at home that would be suitable. I know it is possible to work it out, but I would rather just have a stock of different specific ply wools at home to use when I want. Grrr!
A long time ago when I was still knitting for my children, I found somewhere a list of currently available brands of wool and which ply they matched or were nearest to. A spot of Googling may turn up a contemporary version.
Good thinking Elegran. Another way is to get crafty Nanapug, they are. Read the ball wrapper and see if you can find the tension it knits to, or pick up and read a pattern that uses the same yarn to find the information you need. Failing that, most Double Knit yarns need size 8s and size 10s, most 3 ply yarns need 10s and 11s or 12s, so the number of stitches required to produce a square of 2" or 4" will help you to work it out. They don't tell you the ply so you have to buy the yarn specified, they hope.
Of course if it does not knit up to the size square specified then you just change the needle size, one size up if too small, or one size down if too large, as needed to get the tension specified. It takes time but it is worth it to have a professional looking garment at the end of all your hours of work.
It also helps to buy an extra ball of yarn because if you need to use a smaller needle it may take more yarn to knit it up. Just remember to read every word of the pattern and make a note of any changes needed to achieve the result you want before you start to knit.
Sorry if this is more information than you needed but I can only guess at how much you already know.
Thank you all, especially Cyril for the helpful suggestions. Must admit I have only recently started doing tension squares (since I went on an amazing day course at John Lewis) and find I always need to reduce my needle size. Wondered why all my stuff knitted up large!! I will go for it though and make more effort, then perhaps I can use up some of the beautiful wool I inherited from my Aunt too.