Use your country mobile telecommunications database and go to a location where that band is the only band available from your directly or indirectly chosen mobile network operator? 
Seriously, there is probably a standard or non-standard AT command to get that. However if it’s non-standard then it will be modem-dependent (not a problem at the moment as only the BM818 is being supplied).
The other thing to be careful of is that a cellular modem may present as multiple /dev/tty* devices and you may have to know which one to feed the AT command to.
In addition to worrying about which frequency/band is in use, you will (first?) have to pin down which xG is in use. I think there are AT commands to force use of a particular generation. Again though, I don’t know how standardized that is.
You need a good command reference for your modem but for the BM818 I fear it might be in Chinglish.
The only relevant command that I have played around with is the one to get the signal strength (on my 3G USB dongle), which isn’t going to be useful for this table but may be useful for people who are having problems with getting a (strong / reliable) signal.
Clearly if Purism wants customers to complete this table world-wide then it is in Purism’s interest to provide additional hints if they are able to.