So you've lost the chain slack gauge, or didn't get one with your bike.
You can make one!
Here's a picture of the whole thing.
and here's a closeup of the business end.
Peter Paulovits in Germany reminded me that most of the world is metric, and supplied the appropriate numbers:
|A = 33 mm||1.3 inches or 1-5/16 "|
|B = 43 mm||1.7 inches or 1-11/16 "|
|C = 53 mm||2.1 inches or 2-2/32 "|
And here's how to use it.