I've flown on a flight with air marshals. They were obvious.
They were visiting with and boarded with the crew, before even the first class boarding was announced.
They were wearing slacks, knit shirts and sports coats. Very preppy.
They had military haircuts - actually "looked" like Marines if you understand what that means.
They sat in the second row of first class - I assumed so they could protect the aircrew cabin and flight deck.
As I was flying first class and entered first once boarding began, I watched them... as they watched everyone else who boarded.
They did look very capable, but they were very obvious.
Also, I asked a flight attendendant about 1/3 of the way through the flight and she confirmed quietly that they were air marshalls.
Finally, I must note that although they were certainly armed, I saw no sign of their weapons.