Hanarei
Rawr-ness
Keep in mind that servants as a whole kind of exist as otherworldly beings. They aren't the same as a normal human beings which could explain the ease of servants being able to detect other servants. I mean really if everyone has Mana though, it doesn't really require needing to sense it to drain it from people. You are sure to get some, just how much depends on each person. You do have people like Shiro as well who while mot a Magi during the series formally does have Mana. Its not incredible or anything but its there. If his adopted father didn't teach him chances are he would of still had the same Mana as he posses (ignoring other elements to it). There are likely others like Shiro out there that likely do have higher levels of Mana they just don't have any training (and in many cases likely have no ability to actually utilize it even if they did). Granted you probably would just dismiss someone that has low potential though can be a Magi as likely being a Master since your going to expect the most talented of the family would be taking part rather then some weakling Magi.Yes, but servants can sense leylines. They can also consume the mana of living organisms to sustain themselves somewhat...which does suggest they can sense mana to a certain degree (that's how I assume they sense each other). In that case, it would make sense for them to sense command seals, if we assume command seals to be like giant packs of spare mana...which I kind of see them as.
But I am partial to it being an ability , rather than something given, as I can't remember a single instance of a Servant recognizing a master on sight other than their own
The command seals I just don't picture them being normal so you wouldn't be able to really detect them. They are in a way attached to the grail so likely they don't possess much power in themselves, they likely more function by giving the command and thus it sparks the grail to act on their behalf to cause a powerful effect. In a way think of it kind of like a TV remote, sure you have the power to control and change the channel, but all the power and everything that makes a TV work is in the TV itself.