Apart from the genetics, there are some specific visual differences. I'll deal with the genetics second as identifying what you have determines what you do next.
* Aus Yellow (AY) cannot have a purple or lilac breast, so any yellow birds you have with non-white breasts are Euro Yellows (also called yellowbacks, YB).
* Pure yellow in the back Aus yellows are rare although some breeders are trying to breed out the splotches and so produce a "pure" bird colour wise so that may become harder to use in the future. Most have splotches of light green / dark yellow through the wing and back feathers. However, it is possible for them to be almost pure and while Euro yellows normally have an even olive tinge to the yellow over the whole back that can sometimes be a bit inconsistent. In general a WC Euro yellow will have no olive tinge at all, in fact it even disappears usually with a lilac breast.
So, how do you tell? Only by breeding unless it's obvious from the above. As the euro yellow is dominant *any* pairing with a normal bird will produce Some YB birds. If the cock is YB then all the cocks will be SF YB and all the hens will be YB, in this case the green back becomes pastelly and the black line around the head becomes blue. The presence of SF young in the clutch proves your bird is at least YB. If the hen is YB then all the cocks hatched must be SF.
The difficulty is, if yellow young appear you can't be certain that the partner isn't split AY. The clue is the SF young. If all the young are normals from a yellow / normal pair and you have both sexes in the nest then you do not have YB in that pairing you have AY.
As to the offer, I'd be hesitant to trust anyone with swapping out your birds in this generation. Do some in-house testing first. If you have a new mutation you want to keep control (and glory

for yourself).