Good test. Better yet, if while sewing full bore you pause to get a new grip on the threads and let them go momentarily slack, especially if long, before the stitch is drawn in tight, and they stick fast after even a moment's respite... Anyhoo, who ever thought that waxing your threads makes it *easier* to get things sewed was nuts, not a shoemaker, using bad wax--or using beeswax. You can't sew slow with good wax, or else each stitch will glue itself before you ever get it drawn in tightly. If your wax works fine while sewing methodically slow, it isn't very good wax
Another good test is to watch the holes where the thread's passing through. If there's any accumulation of wax "crumbs", or build-up, your wax has too little pitch in it to adhere to the thread properly. Don't forget the pitch, the other constituent ingredient. The "sticky" part if you will. If making your own--and since Vesta Pech is out, there is no other option is there?--when pulling the wax to bleach and further mix the ingredients, the wax should support itself in a long filament as the pieces are pulled apart like taffy. If it gets but only so thin, then snaps off, it's too brittle for the ambient temperature. IOW, you can know if the wax is wrong before you get so far as making threads and sewing.