I worked at a pizza place for a few years, and when purchasing pepperoni pizza by the slice you need to understand there are two ways the pizza guys can accomplish this; the correct way and the "other" way. The correct way is to add the pepperoni on top of the cheese then bake. The other, or "incorrect" way is to add the pepperoni after the pie is removed from the oven. If the pie is still oven hot, this is not the end of the world (but still not nearly as good). But the true sleaze way is to add the pepperoni to the cold or room temperature slice before warming it up in the oven. I understand some people like it this way. For a long time this was my go to method, but the correct way is many orders of magnitude better.

You may be thinking the "other" way isn't so bad. Here is the problem (I used to think this obvious but people just don't know), while baking the pepperoni will start to cook with the melting cheese. Each little pepperoni disk has a ton of delicious spiced and cured fat that will melt and mix with the cheese (and the sauce to some extent) to create something magical that you don't get after the pie is already cooked.

My recommendation is to purchase a full pepperoni pizza rather than slices. This isn't always possible given time and money constraints.

So you're at the counter of a pizza shop in want of a slice with pepperoni. How do you tell what type of pepperoni slices they have? If you see no pies with pepperoni they will add the raw roni to the slice and then heat it up. If you see a slice pie with pepperoni, take note of how the ronis are sitting on the pie. If they look like red plastic disks on top of the cheese, then they put the pepperonis on a room temperature pie (note, the use of plastic is not a descriptor of roni quality, a great roni may still appear this way on a cold pie). If the roni is slightly cup shaped, but not exactly sunken into the cheese, then they put the roni on a hot pie (this is acceptable in dire need). But if the roni cupped, sunken into the cheese, shrunken a bit and has crispy edges, then you have a winner. You'll also notice a certain amount of oil in the roni cup. This is a sign of true craftsmanship.

TL;DR - this pic made me think of the joy and folly of pepperoni slices.