Just play around with sizers man. You’ll know quickly enough if she enjoys it and wether she sees the fun in going big
This is a great recommendation, one I'd almost forgotten was one we'd tried as well. The "rice test" as it's known, but something such as Plasticene or Play-Doh is a decent alternative. The idea is that you have an amorphous material of whatever volume that you can shape to suit. If you happen to buy a larger bra (I wouldn't spend a lot of money on something which is just a wild guess at this stage) to accommodate the extra fill, that might even help them be less 'silly' for want of better phrasing. Meaning, stuffing more volume into an existing bra designed for a more modest volume could end up just spilling out all over, and not in a good way. You don't want your wife/gf getting frustrated with the temporary overfill experience.
And no need to go out in public. Just fill them up, then hang out at home, watch a movie, have dinner, whatever. Although one time we did go out and clean the inside of my car, that was a bit amusing. Her observation when we were just kicking back watching TV with her ad hoc fakes was that she had completely forgotten about them, until she got up to walk around, then a) she noticed the extra heft, and b) she found herself bumping into things.