how can you tell if two people are having sex

Why would you care?

There are DNA tests now available in drug stores. I bet that you could take swabs of each persons bed, pillows and undergarments and send them into a lab for analysis.

You should listen out for two minutes of something that sounds like spatula thwacking, a few whimpers, a loud exclamation of "TALLY HO!" followed by a whiff of a good quality cigar.

Only if you're British. Us Krauts we jodel in high pitched voices.....it's rather nice when it's in sync (ideally).

In the UK, the lady ahould always yodel first.

As you can see from my video above, it's also the case with Krauts, but
sometimes the yodeling is crafted to perfection so it will be a nice duet

how can you tell if two people are having sex

By the smell.

(Often of fish that's gone off.)

If you ever see them playing leapfrog naked they could be having sex.

It's all a matter of the feathers and how they move.


Kidding. Maybe that the room is quiet. They know you're there.

They both come out of the same bedroom in the morning, and one says to the other, "You were great last night." And the other replies something like, "Oh god, so were you."

Or, when they mistakenly believe that you are not within earshot, you hear one of them say, "Do you think capn is on to us yet?" And the other replies, "I doubt it. I saw that he started a thread on able2know, and it seems to indicate that he lacks enough evidence to confirm his suspicions. So, are we on tonight?" And the other says, "You bet!"

Buy a pregnancy test. Lay it in front of the bedroom door when they are getting it on (at least, when you suspect they do). Interpret their reaction when they discover this test upon opening the door.

As an added bonus, you'l also learn whether it's time to invest in diapers and other baby accessories.

PS. Obviously, this only works in such encounters that could viably result in pregnancy. In other types of couplings (ie, same gender, vasectomy, or just statistically non viable age brackets) said test will probably induce more laughs than anything else, but it might still result in you learning whether or not they are performing indoor sports in bed together.

Please note that it will also be perfectly obvious to the persons in question that you are - at least potentially - aware of shenanigans happening in the bedroom, but since your only restriction was 'without catching them in the act', this is a valid response.

Good luck!

Are they concerned you're "flying solo"?