Hi Chucky, any chance you can join us today? We'd be glad to have you, if so!
As far as knowing what mods you need (or are missing) when joining a server goes, it could be better handled by the programmers. Even now, if I have an addon issue joining our own SimHQ server, there are few obvious clues as to what I need to add/enable to make things right.
At the most basic level, you'd look at the list of addons for a server on the Arma3 server browser screen, and then go manually find all those and add them. Not ideal. Better (as we do at SimHQ) is to point players to links or a bundle of addons that include everything they need.
Whitelisting, that's likely going to come down to the owners of that server basically "allowing" you to join and play. I've never done that, so I'm not certain... Just my guess.