A prefix is a group of letters that have a particular meaning which appears at the beginning of a word.
The prefixes are affixes that form words from a morpheme that precedes the radical. Thus, they modify their meaning but, generally, maintain the grammatical class to which they belong.
You can double check online if the person isn’t sure. Based off of what you find you can make a pretty good assumption to what your answer will be. This is what I do at least. Hope this helps