Coverage checkers are a quick and easy way to see if cellular coverage is available at a site.
These are a good start.
For an important site, a site-survey will more accurately show signal levels and which achievable speeds can be expected.
In the UK, a quick check using he site’s postcode can be made at Ofcom’s website.
You can adjust the coverage check for indoor/outdoor and 4G or no 4G.
Most mobile operators will have a coveage checker on their website, for example in the UK :
Outside the UK, Opensignal provide 2G, 3G and 4G network availability, stats and speeds experienced on all networks worldwide.
This is available for Android on the Google Play Store. and in the App store for Apple devices.