On a guide trip this past summer I had 
a group of guys that got permission to go on an exclusive golf course to hook stocked trout. It was kind of a warm up/casting lesson/fish fighting trip, before we hit the river in search of native wild trout. It was actually a really fun day and I cannot remember having such a ball while getting paid! Each of us guides took 2 clients from the group and taught them the casting basics on the side of the pond. It was sort of an unspoken competition between us guides to figure out what flies we could get these fish to eat and get our clients fighting 5 pound rainbows the fastest. Since I happen to have more flies in my possession than most fly shops do, my clientele happened to be in luck, bigtime!
During the initial casting lesson I had tied on a weighted prince nymph since I had a lot of them. The sage brush that surrounded the pond had me a little concerned about losing flies, it turns out I was right about that; after 3 lost prince nymphs my guys were ready to really catch some fish.
I had taken an egg pattern and colored it brown with a marker. It was a perfect representation of a liver pellet that these stocked trout had grown accustomed to eating everyday. I have found carrying around a pack of markers in several colors has really helped me catch a lot more fish. Read about changing your flies color on the river here. Once this fly hit the water, there are 4 words it takes to describe what happened.. Complete and utter devastation! Massive rainbow trout came from every direction to get a piece of this. Now learning how to set the hook, was a whole different matter; but frankly my client had learned to cast a fly rod and hooked multiple fish in a matter of ten minutes, so I was happy.
It seems like many stocked trout are rather fond of corn kernels for whatever reason, so using a yellow egg pattern will also be rather effective for those fish.
Egg patterns are available at any fly shop or online fishing store for quite cheap because they are so easy to tie, so if you are prowling around ponds loaded with stocked fish, I strongly advise you give this a try.