You'll be able to see the copepods moving around on the rocks and throughout the tank. Dont look in the water column, they dont just freely swim around, look on the rocks and on the sand.
Depending on how much live rock you have and how many fish are eating the copepods, you may or may not have to buy more. A well established aquarium of 90g can support one mandarin as long as you have enough live rock or other breeding grounds for the copepods.
Of course, if you only put in 10 and the mandarin wipes them out before they have a chance to reproduce, you'll have to buy more.