1.
Message me for an invite
A "Free lobby" that you have to "message someone" to get into, most legit lobbies will just be join session due to the high numbers of people wanting in, there is no way for a person hosting a legit lobby to invite everyone that messages him. If you actually message the kid, you are more than likely helping a troll to troll another person that has no clue about a modded lobby by having his GT spammed with messages.

2.
Main Gamertag.
If the person hosting the lobby is using a Main GT, its more than likely fake. Ex-High gamerscore. Years gold. big friends list ect.
Why would you host something with your main gamertag when its a matter of time before the entire console gets banned??? Again you are helping a troll with spamming his victim.

3.
Low post count.
normally people that are in the jtag game have been on TTG for a while and dont use brand new accounts!

4.
Pay your way in.
ONLY trust a pay your way in lobby if it is with a VERY trusted TTG member that has extensive vouches. And their threads will almost ALWAYS have some pretty legit PROOF.

its not alot, but its enough to help you out a little i think, or atleast to not have to waste an hour of your life trying to join a kids session thats not even legit!

You can list more clues if youd like for me to add.