Kan Jam gets asked this question a lot, so they made an app to solve this problem. It’s available on Android and Apple. We downloaded and tested the app out for this Kan Jam review and it’s easy to use with no detectable bugs. You can pick who threw last on which team and add or subtract 1-3 points.
If you don’t want to use an app to keep score (sometimes the point of an outside game is to get away from our phones!) then the options are pretty open: call out scores after every toss, count in sign language, use cave art. Just make sure it’s not your notorious-for-number-fudging uncle keeping score.