If you are using mason jars, there are marks on the jar showing volume in ml. Eyeball it and estimate how many ml of yeast you have in each jar. Go to Mr Malty and enter the OG of the IPA you want to make. Enter the date you harvested the yeast. The calculator will tell you approximately how many ml of slurry to use. I always add a bit extra to be safe. If you have enough, you can just pitch it. If not, build a starter to increase your yeast volume.
If you over pitch, it's no big deal unless you really go overboard in a big way. These are only estimates and guidelines for those of use without microscopes and lab equipment.