Tricky: a scammer could also contact both parties individuality at the same time, ask the other party to confirm by PM on Bitcointalk, and it'll be very difficult to spot there's a man in the middle attack going on!
All messages can be forwarded unedited, except for Bitcoin addresses.
It's not so easy to contact both at the same time because they can answer at the same time and then the scheme collapse.
First ask one if they can be the escrow for a deal or the lender for a collateral loan. They fits them anyway any answer.
If the lender agrees to be the escrow, then they copycat his name and create a group with the copycat account (escrow) and the victim (lender) to convince him that is the real escrow account/ member. So the victim (lender) can send the funds to the escrow (copycat) address.
If the lender agrees to fill it then they ask for an escrow,
"since they are newbies here" excuse for example, and they copycat the name of the escrow that the lender choose. Then they make the group with the same goal. I hope that you understand what i'm trying to say.
Best way is to sign a message at the beginning of each conversation.
On TG?! As far as i know, you can't sign a message to verify that you are the real owner of a TG account.