You can check your version from the top right of the menu. Once a day the game will check online for an update and SHOULD notify you of it when you start. You then need to re-use your humble widget download link to grab the installer again.
A better patch system is planned for the future
If you have lost your emails from humble you can get your order resent from this link:
Why 18.5? Why this date in particular? LOT of games getting released or go into EA on steam at that date. Just why?
Is there some conspiracy to game me to death or something?