How can I tell if a breaker uses both phases/legs
Stab-Lok is a PROBLEM
See, for example, What can I do about a breaker that trips frequently on a Stab-Lok panel?
So first read up on Stab-Lok, post a picture of your panel (the pros will recognize it and tell you if it really is a fire hazard, etc.) and determine if this is a concern. If it is a real concern, then dealing with replacement is a much higher priority than hooking up the Sense monitor.
How it SHOULD Work
On many panels (I don't know if Stab-Lok is supposed to work this way or not), a true double (240V) breaker will always hit both legs. The exception is a double-stuff breaker, where it is really two-breakers-in-one. In that case, it normally only hits one leg, and the exception is if it can actually be 1/2 of one leg and 1/2 of the other leg. But that, as I understand it, is only on certain panels where you can have a quad == 4 in one, but really 4 in 2 - i.e., a double-space breaker that has 4 individual breakers, two on one leg, two on the other leg, with the result that the inner breakers can function together as a &quot;double 240V breaker&quot;.
FIRST FIGURE OUT THE STAB-LOK!