Find the radical of a number? What it called

When you addition you say your adding same with subtraction you say your subtracting, but when you have a radical do you say your radicaling it?? Whats it called ._.

The word radical in a mathematical context is a noun, and not a verb, so there is no "-ing" variant.
You would say I'm determining the radical, or finding the radical.

Think of it like the word "cat", you could say "I'm finding the cat", you wouldn't say "I'm catting".