Mion
Always Tired
Tohsaka literally explains to Emiya about the master to servant leak in f/snI never saw that either. In fact the only time mana flowing into a servant is everused is when Kiritsugu did it. But that was just good old fashion technology, not magic. Everything shown in both FSN and F/Z tells us masters have no inbuilt way of detecting other masters.
"A Master that is providing magical energy to a Servant will "leak" magical energy in a volume that is easily detectable to most spellcasters. The leakage is distinctive, allowing a Magus to determine that the Master has a contract with a Servant.