Try pixelmator,or look for histogram in App Store .I don't have Adobe subscription, do you know any other app?
Ok,I usually only need the histogram to check lightning in my pictures,try any of the histogram apps. I can unfortunately not tell you which,as I use Lightroom myself.I have pixelmator on iPad but i can't find a way to do what i want, i attach a photo example of a pear with low light highlighted