How can you tell if you are the first to march on a rss tile?

It is not easy to tell. If you tap Dispatch on the march and immediately get a warning that someone else it heading there then the chances are that they sent first. If you only get the warning after you have been marching for a few seconds then you were probably first. But with the possibility of lag the other person may think they were first. It is also fairly impossible to prove who sent first. It is a minor issue with the game but not one that is easy to fix.