You can see at a glance if the moon looks like

1. it's full or almost full, or

2. new or almost new (when the moon is crescent shaped)

 

What the terminology means

• "Waxing" means the moon is moving towards being 'full', when the moon is the most visible in the sky. 
  If the app says 'Full' it's a full moon day and there is no yoga class. If it says it displays a '% of Full', it's still waxing towards becoming full. There are yoga classes on these days if there is one normally scheduled.
   

• "Waning" means the moon is moving towards becoming a new moon, when the moon is the least visible in the sky.
  If the app says '0% Full