dtrout, i fly extensively. It is easy to spot the air marshalls. for the interest of security i will not say here. but trust me, a three year old child can spot an air marshall (and did, i have a great story).
the air marshalls are not provided by the airlines. they are provided by the TSA. there may be one or two air marshalls on the flight, or none.
I haven’t seen an air marshall on a flight yet, this year.