your question has plagued many a
SH keeper. the only way to find such a fish is to get your booty over to the
LFS and ask them if they have a mandy that takes prepared food. if they tell you "yes", ask to see the fish eat. as you're watching the fish eat, look it over for damage, thinning (it should be nice and fat), signs of disease, etc.
one thing to remember is that a mandy that eats prepared food can suddenly stop for no apparent reason and you'll need to be prepared to feed it live foods. at that point, your
SH will be in direct competition with the mandy for food, altho most adult
SH will ignore copepods and
BBS, which the mandy will eat with relish.
there's a nice article in the
Reefreaders Articles section of this site that may help a bit more.