Testing and proper breakers for a multi wire branch circuit?
The easy way to tell if an outlet is powered by an MWBC is to turn off the breaker(s) known to supply power to that MWBC and see if the outlet's power goes away.

The US National Fire Protection Agency's National Electrical Code does specify that all nongrounded conductors in an MWBCs must be controlled by a double-pole breaker or marked pair of linked breakers.

It's absolutely permissible to link breakers together using that hole - that's what the hole is there for. In most cases, though, it's a better idea to use a unitized double-pole breaker; that way you're likely to get the best possible match and service life between the two individual internal breakers.

Note that it is NOT permissible to link two breakers with different CURRENT ratings together in that manner - all nongrounded conductors in an MWBC must be the same size wire, and so if two linked individual breakers are used, then they MUST have the same current ratings.