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 "double 240V breaker".

FIRST FIGURE OUT THE STAB-LOK!

The answer from manassehkatz (not related, but we do know each other :-) ) is very important, but it doesn't actually tell you what you want to know.

If you have a voltage meter or multimeter, you can very easily find out whether the poles of this breaker are on the same phase or different phases. (Important: if you are using a multimeter, make sure it is set to voltage measurement, not one of the other settings, before you touch the probes to anything.)

To test, touch one meter probe to one breaker terminal and the other probe to the other terminal. If you read 240v, you are properly on both phases. If you read 0v, you have both on the same phase. You can do this same test on other two-pole breakers too to see what it should look like.

I did manage to figure out the Sense problem by looking at the specs diagram for the panel. It showed a set of 2 poles with one that used phase A and another that used phase B.

Sense issue is resolved but now I need to get this panel looked at and replace if electrician states it is a problem/fire hazard.