Store Supervisor Question:

Ask the employees how they're liking their job on a regular basis. Encourage them to be honest with you. Then take action based upon what they tell you.
Offer perks that your employees will value. If health is important to them, give them time to go to the gym and work out. If their family is important, respect the time they may need to send their kids off to school in the morning or pick them up in the afternoon.