The main character(s) are looking for telepathic geniuses to hire or to admit to some school or something. Everyone who respond to the advertisement enters the room one by one, picks up some paperwork etc., but sitting in the room next door is a real telepath and he transmits a message mentally: "if you hear this, go to room number..." So only the real telepaths can hear the message and that's how they identify them.
It sounds like this scene from Alfred bester's The Demolished Man. It's a fairly short (by today's standards, anyway) novel first serialized in January 1952 in Galaxy Science Fiction magazine.
The usual line was assembled in the anteroom of the Esper Guild Institute when Lincoln Powell entered. The hopeful hundreds, all ages, all sexes, all classes, each dreaming that he had the magic quality that could make life the fulfillment of fantasy, unaware of the heavy responsibility that quality entailed. The naivete of those dreams always made Powell smile. Read minds and make a killing on the market… (Guild Law forbade speculation or gambling by peepers) Read minds and know the answers to all exam questions… (That was a schoolboy, unaware that Esper Proctors were hired by Examination Boards to prevent that kind of peeper-cheating) Read minds and know what people really think of me… Read minds and know which girls are willing… Read minds and be like a King…
At the desk, the receptionist wearily broadcast on the widest TP band: If you can hear me, please go through the door on the left marked EMPLOYEES ONLY. If you can hear me, please go through the door on the left marked EMPLOYEES ONLY…
To an assured young socialite, with a checkbook in her hand, she was saying: “No, Madame. The Guild does not charge for training and instruction, your offer is worthless. Please go home, Madame. We can do nothing for you.”
Deaf to the basic test of the Guild, the woman turned away angrily, to be succeeded by the schoolboy.
If you can hear me, please go through the door on the left…
A young Negro suddenly detached himself from the line, glanced uncertainly at the receptionist, and then walked to the door marked EMPLOYEES ONLY. He opened it and entered. Powell was excited. Latent Espers turned up infrequently. He’d been fortunate to arrive at this moment.