Periodic health checks are actually standard procedure for most pet owners... I know I do it for my dogs and rabbits all the time. Every few days I will pick them up to feel weight and make sure they're used to it (even my big dogs) check things like eyes ears and paws, fat around the ribs, teeth & mouth (beak in this case), nose, fur condition, heart rate, gently palpate their abdomen a little, move their limbs to feel for stiffness and make sure they're all around fit.
It let me find something was wrong when one of my rabbits began loosing weight rapidly due to old age and got quite skinny.... But because of the fur I would have never known if I hadn't been putting my hands on her regularly to know what was her normal weight and it also let me know when she began putting weight back on and I could level off her high-calorie diet. She's now a couple years past her "expected" lifespan and still active and healthy.
Also by keeping my animals used to being handled it makes thing easier when one gets hurt. My big dog (65lbs) recently got bit in the leg at the dog park and we had to carry her up the apartment stairs after the vet appointment and she was quiet and tolerant of it. I have seen lots of animals go bonkers when picked up and cause more injury to themselves and others because they're scared of it.
I would say pick up your ducks regularly and feed them treats while they're touching you. Peas are apparently a big hit (just thaw some frozen ones). Find a treat they love and once you know they like it don't let them have any without being very close or handled.
Do this sort of thing occasionally whether or not they look healthy because knowing how a healthy duck feels is more important then knowing how they feel when they're unhealthy... If you know how a healthy duck feels you'll have a comparison for when one doesn't feel well anymore.