Hook Gauge by Yarn

Its often the case where I find yarn with no labels and this method has always worked for me...

Make 4 strips of your yarn...

Pull the 4 strips tight and "hover" over various sizes until you match the thickness of your strip with the width of your hook (or needles)

Like this match...

Tahdah!! I will use 4mm hook with this yarn! 

If you have fun yarn its a little more difficult to hold the "bits" in, sometimes I make my 4 piece strip shorter and tightly roll them in tape, this isnt always the best method for fun yarns but cant hurt to try. 

Your best bet is to search for the fun yarn and find label recommendations. 

Happy Hooking!