Here comes a site that aims to make your life as a consumer much easier: Will My Phone Work
Simple interface lets you select your Brand, Model, SubModel (if any), the country you're from and the carrier you want to use.
If you click on the search button you'll then be provided with the results of the carrier/phone combo you've select and see the services that will be working.
If you unlock your phone to be used with a different carrier, don't forget that you'll need the APN settings for that carrier and you can find all that information here: databases.androidcoliseum.com
Source: Android Police
EDIT: Good to note that within 24 hours of emailing them about missing the LG G3 d852 model they added it (and other variants) in! Great work