How to know how much milliseconds delayed video from the user?
Sometimes it's not "miles", but seconds... Using RTMP delay in seconds from 1 to 5 usually. If You clarified, why it is necessary - perhaps there would be some more accurate solution.
And generally, judging by specifications RTMP Protocol, there is a PING packet. You can try to catch them using a sniffer and measure the delay or instead of RTMP server to stick some kind of plug (the plug) which would be sent instead of the live ping packets and measure the response time...
*this is more "thinking out loud" as the details of the issue are not known and formulate some kind of "good solution" quite difficult (to me).