How can you be 100 percent sure someone is actually your real friend and not just using you?? my friend? Or suppose to be friend says that we should be frenemies is that a good or a BAD sign

A real friend puts up with you through fights, depression, complaining, and changing plans at the last minute (that's just for starters). You share every thought and emotion.
You just know! Of course you have to do the same for them.
How old are you? This is the sort of question a young teen would ask. Frenemies might just be a cute word of the moment while you fight. We aren't there to see you two in action. If you feel used, then I'd say you aren't friends.

Yes I would guess by the language and way it is written a preteen most likely.

A real friend will be there for your regardless and will not be playing silly friend/enemy games.
But at 11 or 12 many children don't know the real meaning of friend