The two-checker rule
Run the network's own postcode checker and Ofcom's independent one. Agreement means confidence; disagreement means your trial month decides. Weight indoor predictions heaviest — indoors is where you live, and where every network is weakest.
The one-month trial
Buy a rolling SIM on the target masts— via the budget tenant for pennies.
Live on it for a full week— home, work, commute, the supermarket queue.
Test the rooms that matter— bedroom, kitchen, home office; not just the doorstep.
Decide— pass: commit; fail: you've dodged a two-year mistake for pocket money.
The yearly re-check
Masts get built, upgraded and re-farmed constantly — the VodafoneThree merger alone is redrawing maps. A postcode that failed last year can pass this year. Ten minutes, once a year, before renewing anything.
Pro tip: Moving house? Run the checks for the new postcode BEFORE exchange — signal is easier to verify than to fix.