Try to pay attention specifically to that sixth degree which distinguishes between them, another idea play the given scale on an instrument and check that sixth degree

Yep, as Yooval suggested, just sit at eg. piano and play around listening to the natural vs b6. Just play in the top half of the scale and even sing along.
Another thing that's good to do is to use a different (cough) app that allows you to compare just those two for a series so you can practice that specifically.

Yeah, actually in the gym under exercises, it only allows me to pick a minimum of three scales to compare. I'm using it anyway because I'm working on distinguishing between natural minor, jazz minor and Dorian.

I'm finding that just paying attention to the interval between six and seven is the trick, for me.

I didn't realize that was an option! Good to know!

I'll echo Yooval here, count along with the scale tones, pay particular attention to the 6th tone, ask yourself: was it a half or whole step?