Comparing the cost of keeping an old phone plan with switching to a cheaper provider that has weaker coverage

My old mobile plan costs more than the promotional offers I keep seeing. The cheaper provider claims good nationwide coverage, but several areas I visit have unreliable service even with my current network. Saving a few dollars isn't worth losing calls while traveling for work. How do you assess real local coverage before switching—prepaid trial SIM, maps, neighbors' experience—without trusting marketing coverage percentages?