How to decide if something’s a good fit
There are actually two ways to tell whether a choice is one that's well-suited for you:
Look for a better fit.
Test it.
I think we can agree that convincing yourself that—if you can find a better fit—it obviously means your original choice wasn’t ‘a great one,’ is probably a bad strategy.
The folks who spend their time looking for a relationship, only to jump ship the moment they find someone more attractive/funny/interesting/etc. are certainly in this camp. As are all the folks who switch career goals (or majors) every quarter.
On the other hand, testing a choice—jumping in and getting a feel for it, long-term—is a radically different, and potentially higher-yielding approach.
Because experiencing it—as opposed to thinking about it (or making choices based on external factors)—allows you see things thoroughly, holistically, and with nuance. And it’s what allows you to make decisions not merely based on logic, but based on feelings...gut reactions and subtle cues.
It’s not nearly as easy or comfortable or straightforward as the former option. Certainly. But it enables you to live with the reality-based conviction that your choice—whatever choice you make—was or wasn’t the best choice for you.