Most any pants or shorts, 1 size larger than normal, will accommodate diapers fairly well. Also, I found anything with extra creases or pockets (like pleated pants) visually break up the area around your crotch, which helps hide a diaper quite nicely.
One other thing that helps hide a diaper is a good diaper cover. Leaks are a dead give away, so containing them before it becomes outwardly noticable is a must. I personally prefer Gary Activewear PUL pants. I wear them inside out, so the nylon layer is on the inside. This allows the cover to absorb some of a leak, while also helping me to notice it a little faster. PUL pants also muffle diaper crinkling too, furthering one's stealth.