How to tell whose squab is whose when fostering?

[Journey]

If you have a pair foster an egg or squab from another pair, other than putting them with foster parents that can't possibly produce that color squab, is there any other way of safely "marking" the squab so you can tell which one it is?

You can obviously mark the egg but once it hatches there are very few ways to tell squabs apart...

[AdamArcher]

If I ever have to move a single egg into an established nest I try to use a nest that I know will produce different coloured young so I should be able to tell when they feather up.

[3DLOFT]

Normally i use a marker on the egg, and when it’s hatched i mark the squab nail or under the foot