As said, it's important for them to be healthy when you get them - you can try training them to eat, but know it is a risk that they won't convert fast enough, and the timer is basically how well fed they've been prior to you getting them and how free they feel to explore the tank.
If you can get it at a local shop and see it, it will be better, but if you're getting from a reputable online place (or one in a WYSIWYG section where perhaps you can confirm it's been eating), it could certainly work. If you are able to observe it but not see it eat, you want to see foraging behavior - usually prowling around and occasionally pointing its nose down in pursuit of something on the substrate or rocks, and of course you don't want the mid and rear section of the center of the fish to look thin - their head area doesn't expand or contract much with weight, but their mid section shows underfeeding pretty clearly. If they aren't hunting for food or are swimming at the top or hiding all the time, they aren't engaging in the behavior they need to realize prepared foods are good to eat, and in my experience it's unlikely they'll be able to live long enough to convert.
If you get one without confirming it eating, ideally, you want a lot of life on your rockwork. Their normal diet includes fan worms, small bivalves, and other rock dwelling (but not copepod) creatures. If they have some food they can get in the interim before they recognize the food you're offering as food, that will give them a lot more time to learn. When offering food to transition them to, offer frozen, especially in chunks just a bit smaller than their beak, and while I haven't had specific luck with it, offering masstick on a rock or on a clam shell (or just frozen clams on the halfshell) could be a good first food for them to start eating. I don't have proof, but I believe it's also better to feed with the powerheads off until they are eating prepared foods directly from the water column.
In my experience, if you can get them to the point where they will start eating some frozen food that's resting on the rock work, you're basically in the clear as they should only take a day or two to recognize it in the water column.