How do I know how full a server is?

[Niwashi]

I figure this, the lower the pop the last likely you will get in a dungeon party.

For Duty Finder, it doesn't matter what the population of your server is, since that draws in people from other servers as well.
For forming a party yourself by finding people from your FC and/or linkshells to join you, it's the size and activity of your FC/LS that matters rather than of your server.
But for using Party Finder to form groups, this can be true. High-population servers will generally tend to have more PF activity.