But how would one know they are on the fence?

By measuring water parameters?
You can have the cookie back but Im keeping the pizza!
How do you know they are on the fence? You don't. Assume they are and do another WC. Water parameters? What are those? Do another WC.
This sound familiar?
Rays aren't stupid, if they can let you know they are hungry they can let you know that the water sucks too.... OR you can just do as I do and assume the water always sucks. WC.... I see poo! WC. Bastids! Fresh poo. WC again. Does it suck? Yup. Does it suck more to have rays on the fence? YUP. IS there nothing with a bigger suck factor then watching them die? No. Except when your truck gets stolen.... But details on that would be a derail......