Flash - test in several locations/at several times of the day. If you get good connectivity/data speeds, it's a keeper. If it doesn't work (or if you get really poor/no connectivity immediately after flashing it), flash another, etc.
There is no 'scientific' way to do it. You either find out what works with your carrier through trial & error, or you try modems other people who use your carrier find work well with that carrier.
Stay away from flashing RIL's other than a last resort/if you end up trying pretty much every modem you can find & your connectivity is still poor. If you flash a RIL trying to say, get 7Mbps downloads when you already get 6.8Mbps, you may end up borking your connectivity completely. I have seen people on XDA do this (both flashing a RIL for a potentially insignificant improvement in connectivity if, and borking their connectivity as a result).
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Most retarded thread on XDA in 2013