How do I determine if an RF signal is transmitting? Re: Weather station

I dug out a couple of my older weather stations this past week and installed batteries in the base units and remote sensors. None of the base stations show connection with the sensors.
As the highest frequency my o-scope can read is 125 MHz, and the sensors xmit in the the 400 and 900 MHz regions, how can I determine if they are transmitting?
I do have a chinesium SDR I bought a couple of years ago, but I haven't able to get it to work.
If I do get it working, will it be sensitive enough to pick up a signal?
No, I do not have schematics for the sensors.

 

@Harold Kapp,
Not pairing like in Bluetooth, where a code must be entered for the connection to complete.
Usually, the remote sensor (temperature, Humidity, Wind Speed and Direction, Rainfall, etc.) needs to be powered up first and when the base station is powered up, it will connect to any signals matching its protocol.