These two species are very similar to each other and differ from Polyommatus icarus because in Polyommatus icarus always (with the exception only of Polyommatus icarus f. icarinus) there is a mark in the cell of UNF.
Polyommatus escheri has one generation while Polyommatus thersites has up to two, depending on altitude.
In males of Polyommatus thersites there are androconia in the discal band of the UPF and thus they are easily distinguished from males of Polyommatus escheri.
The fringes of the wings (as long as they remain intact) are characteristic and can be criteria for separation (as noted below).
In male Polyommatus escheri there is a clear black band, in the UPF and UPH, which is absent from the male Polyommatus thersites.