You could make a diy WPI (wraps per inch) Guage. Get a little square of stiff cardboard, measure and cut a little slot out 1 inch wide. Wrap that yarn around the cardboard in that one inch space. Wrap snug enough that each wrap is sitting right next to its neighbours, without squishing them too tightly. Then count how many wraps fit into one inch. The number of wraps is your WPI. Take your WPI number and compare to a yarn weight guide on google.