It's a little silly, but I can tell by whether or not by looking at the background. The image I'm currently trying to save has no background, making it transparent, so when I see it has a white background in the photos app, I know it saved as a jpg instead of a png. Also there's some artifacting. To compare, I save an image with no background from Procreate, which shows a black background, indicating that there's nothing there.
So, if there's a white background + artifacting = jpg, which this image certainly does contain.
Possible note worth mentioning, prior to saving I actually changed the size of the canvas's dimensions. It seems to have a factor in how saving behaves, especially in smaller sizes.