Scenario: You are sharing a house with other people, be it friends or distant relatives. How can you tell that two people are getting it on, without catching them in the act.
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.