How can I tell if one iPhone receiver works as well as another?
You can do some work to narrow down performance, but you'll need carrier help if you find that there's an issue with one device.


Use Safari to do a speed test http://fast.com on both devices - even if you are using different devices, different cell towers (just because two phones are in the same place - they can negotiate different frequencies, different towers or get different levels of service from the same tower at the same frequency).


By focusing on measuring the flow of data - you can over time know if one device has hardware limitations or hardware issues.

Next, put each device in field test mode: Dial *3001#12345#* - if you can see in the menu bar a number like -76 then record those for each device and then call your cellular company to help determine if you have coverage issues or need to update your phone's settings or software.

_Note: not all carriers have a useful field test mode. See this and this and that for more details.
Basically, each cell tower is a time share. It juggles all the devices in range - tells some to be on LTE, others to be on 3G, others to be on EDGE. Unless your phones don't work at all - the cause of differential service is almost always a device that's stuck and needing reboot or a congestion issue on the carrier end. Apple can also run diagnostics if you believe the hardware is faulty - but that's usually a low chance of being the cause.