How to test Honeywell aquastat L6006C?
B is the normally closed contact, W is a normally open contact, and R is common.  So in the initial state, you'll measure continuity (or a low resistance) between R and B, and no continuity (or infinite resistance) between R and W.

When the temperature rises above the set temperature (+ the threshold), the contacts will switch. So you'll measure open (or infinite resistance) between R and B, and closed (or low resistance) between R and W.

To test it with a multimeter, you'll want to use the continuity, or a low resistance setting on the meter.