For example, I was looking at NFL fantasy stats here: https://rotogrinders.com/projected-stats/nfl-qb?site=draftkings
And just by trial and error I discovered you could use "&date=2016-X-Y" to get the stats for a specific date.
Is there anyway for me to have discovered this other than by trial and error? I've been poking through Chrome's developer tools but can't seem to find anything.
URLs are usually interpreted entirely with server side code, which isn't available to clients.
Reverse engineering from the user interfaces websites provide is normally the only way.