I used to own a Jenday...
The differences are subtle but obvious when you know what to look for. Here's a comparison of a Sun Conure vs. Jenday Conure, side by side.
As you can see between them they look pretty similar but definitely have difference in plumage.
-Jendays have a ring of red around the eye
-Jendays will have am orange/red belly that wraps around the head but none on the head, it stays pretty yellow
-Jendays will never usually have any yellow come in on the wings.
-Suns are much more yellow but can have varying degrees of red, usually around the face
- They lose their green plumage on wings as they age.
I had my Jenday for 5 years and he never had a single yellow feather develop on his wings. They stayed green. The only way to know for sure what he is is watch him as he ages.
If he gets lots of yellow on his wings (as pictures above), he;'s a Sun. If he stays mostly green with a little yellow, then he's a 'Sunday' hybrid between the two, which happens. If he stays all green on the wings, he's a Jenday!