If your snake is a year old it probably can handle fuzzies just fine. Hoppers are a bit bigger and if your snake is only about 15" long it might be too much meal for it. I have a 10 month old female who will be moving up to hoppers, she can easily put away 2 fuzzies in one feeding and it barely makes a lump. When my snakes can eat 2 or more food items in one feeding and the lump is small it's time to move up
That is how I decide but others may have methods that work well also. Hopefully others will advise on this too.
Alicia