That is really hard. As a parent you are always concerned about that kind of thing. Your six year old should be able to tell you why, or where he 1st saw it. Make it a game so he doesn't get defensive.
I would play a game with daddy and mommy and him called 1st time. "daddy when was the 1st time you ..." then start with things like:
Played baseball
Ate watermelon
Saw mommy (or daddy - short version!!)
Saw micky mouse
Saw ________ (add a friend or family name)
Saw someone hugging
Saw someone tickeling
Saw kissing on cheek
Saw kissing on the hand
Saw kissing on the forehead
Saw kissing on the lips...
THEN HERE IS THE KEY!!!!!! Keep playing the game!! Let him come up with "the 1st time" things to ask you throughout the game, and make sure it is ended with, "Wow that was a fun game, I really liked hearing all your answers, and you asked me and daddy some pretty good questions too. Should we play this again sometime?"
Hope this helps a little, my son is 4 and we have had to deal with some similar situations. it works as "when did you 1st hear..." too. We found out just who was talking the way our son started talking, and needless to say, he spends way less time with that person.
Good luck, and don't draw conclusions or worry yourself until there is proof of something to worry about. You will only drive yourself crazy.