How can you tell if a mobile phone has good reception before you buy it?

Paul lives in a signal black spot where his wife's smartphone still works. Is there a way to find if phone has good reception?

I have a Moto 3 smartphone and my wife has a similar earlier model. We are right on the edge of reception from EE. I can just get a very weak signal if I attach my phone to a selfie stick and lean out of a window, or walk up the bank behind the house. This is not ideal.

Away from home, sometimes my wife's phone can get a strong signal whereas mine can't get a signal at all. This made me wonder if there was a measurement to assess how good a mobile phone is at receiving signals in areas of poor reception before you buy it " and if there is an easy way for an ordinary punter to understand it. Paul

Phone manufacturers and others can and do test their phones, usually for certification purposes. The performance test results you want, if you can get them, are the Total Isotropic Sensitivity (TIS) value for reception and the Total Radiated Power (TRP) for transmission.