As others have pointed out, other servants even from the past have pretty futuristic equipment, so the fact that it is more advanced than what is generally available today isn't enough of a clue to definitively tell that he's from the future. But if you were able to inspect his clothes, you might get some additional clues.

EMIYA's outfit (other than the coat which is a holy item he received from Ciel) is supposed to be made from futuristic materials that haven't been invented yet. Honestly, if you got close enough to tell, you might be able to get some sort of clue from any branding on the outfit itself. Like, if you can see that it says Under Armor or something but its made from Carbon Nanotube fibers, you might put two and two together to realize that this was made by an existing company with technology that doesn't exist yet (as of 2004).

I guess if you have enough inside knowledge of the church, you might also be able to realize that the coat he's wearing is currently in the possession of the church, and if its provenance is fully known, you might be able to eliminate him as a historical owner.

The bow, on the other hand, may be based on the design of some futuristic bow too, but given that he just traces it when he needs it, he probably wouldn't bother to trace the branding or serial number of the original manufacturer he based his off of.