[Freeswitch-users] How to check if user is behind NAT in a programmatic way?

Hello I would like to allow direct p2p media (bypassing FS) between registered SIP users, but only if one of them is not behind NAT. I can't force bypass_media=true for all calls, because many of users are behind NAT and this will break communication. I only want to bypass_media for users where it's possible. Is there any way of checking this in a programmatic way? I've checked call variables, SIP registration info from database but I can't find any related variable. Any suggestions?