Well, I am certainly no expert, and i'm not as experienced as the other members on this forum, but if..

1. You have only the tip of your tongue on your palate

2. You don't engage the posterior third of your tongue on the palate

3. You focus the force of your tongue on your teeth instead of your palate

4. You swallow incorrectly (use buccinators instead of using tongue to push food down)

5. Only mew in the day but at night, your mouth is wide open (you can tell that you are doing this if your mouth seems dry in the morning or your pillow is wet due to drooling, if you sleep mew properly you should never drool.)

Then you are mewing improperly.

Im sure there are more signs that you can be mewing improperly but these are the ones I can think are the most obvious signs.

I was looking for answers like you have eyebags or something like that, but good list. I want somebody like @eddiemoney to inform me on this.