Determining Secondary Cellular Provider(s)

We currently have a handful of Sierra Wireless dual sim modems with Verizon sims. We have often found ourselves in locations where the Verizon coverage is not ideal. In trying to identify a secondary provider, I have found the easily accessible coverage maps to be useless. Does anyone know of any ways I can try to identify an ideal secondary provider? Is there possibly a device we could use for spectrum analysis and identify the ideal carrier?

Thanks for any suggestions!