You can achieve "envelope detection" with a single diode, slightly better with two, plus a small capacitor and a resistor.
The diode needs to work at the frequency of interest - 2.4 GHz in your case. If this is of interest I can comment further on components.

Connect this to the coax output from a quarter wave antenna (ideally a dipole with either a sleeve 1/4 wave on the coax or a ground plane). At full power an ESPxx may deliver 10's of milliwatts into its transmit antenna. An analog meter on the diode detector output set to mV or ideally microvolt range (eg 2.0 mV range on some meters gives you 2000 uV full scale. An analog meter may be better than a digital one. Then move receive aerial VERY near transmit aerial and try turning transmitter on and off. Move it away to see distance effect.

Test on a cellphone to confirm that it works - you may need a different length aerial BUT at almost touching range it may not matter enough.

More input available if of interest.

NBNBNB: Get an oscilloscope! - Even an old terrible one or an AliExpress cheapo will transform your experimenting.

This is a prettier way of doing the same thing.
I've got one of these coming from China - due today (it's 2am here now :-) ). This cost me about $US4 with free shipping and about 10 day delivery time. YMMV. Mine is from here BUT that's NOT a recommendation - it may or may not work (diode may be flaky, but shouldn't be) and there are other sellers.

The P2A is a double diode - one connects input to ground, and the other of opposite polarity connects input to filtered output.
The pink component after the diode is a capacitor to ground and the black 103 is a 10k resistor load to ground.

You can achieve "envelope detection" with a single diode, slightly better with two, plus a small capacitor and a resistor.
The diode needs to work at the frequency of interest - "MHz" in your case. If this is of interest I can comment further on components.

Either feed the signal directly to the diode input, or via a capacitor.
In your case the signal is "captive" BUT you can do this via RF coupling.

If you want to determine if the ESPxx is working you can detect it's RF signal. You could turn it on and off under program control every say 2 seconds so you can easily detect the signal.

Connect the input to the coax output from a quarter wave antenna (ideally a dipole with either a sleeve 1/4 wave on the coax or a ground plane). At full power an ESPxx may deliver 10's of milliwatts into its transmit antenna. An analog meter on the diode detector output set to mV or ideally microvolt range (eg 2.0 mV range on some meters gives you 2000 uV full scale. An analog meter may be better than a digital one. Then move receive aerial VERY near transmit aerial and try turning transmitter on and off. Move it away to see distance effect.

Test on a cellphone to confirm that it works - you may need a different length aerial BUT at almost touching range it may not matter enough.

More input available if of interest.

NBNBNB: Get an oscilloscope! - Even an old terrible one or an AliExpress cheapo will transform your experimenting.

This is a prettier way of doing the same thing.
I've got one of these coming from China - due today (it's 2am here now :-) ). This cost me about $US4 with free shipping and about 10 day delivery time. YMMV. Mine is from here BUT that's NOT a recommendation - it may or may not work (diode may be flaky, but shouldn't be) and there are other sellers.

The P2A is a double diode - one connects input to ground, and the other of opposite polarity connects input to filtered output.
The pink component after the diode is a capacitor to ground and the black 103 is a 10k resistor load to ground.