I have older dogs and have been supplementing their dry food with wet food.
They normally go after the food until its gone... I bought some new cans of the same food
And they suddenly turn their noses up at it... can dogs tell there is something wrong with the food?

Check the expiration dates and smell it for yourself. Dogs have been known to eat things that were no good so it may not be a good indicator of bad food. Try something else and see if they will eat that. If they are acting fine otherwise, don't worry just yet. If you are seeing signs of illness like vomiting, diarhhea, or lethargy call your vet. Don't let them skip more than a couple meals though without touching base with your vet.

Can you tell when something's wrong with your food. That's your answer.

If they are eating grass when you take them outside they need to see a vet because often times eating grass for a dog is a sign of sickness. If not change the food, but heads up changing food will cause them to have disgusting poops because their bodies have to adjust to the food.