Telling the difference between the Spotted Fritillary (Melitaea didyma) and the Lesser Spotted Fritillary (M. trivia) can be difficult but one reasonably reliable diagnostic feature is the presence or absence of orange markings on the body of the adult butterfly. By and large, the Spotted Fritillary has orange markings on its abdomen, whilst the Lesser Spotted Fritillary does not. Given that you can expect to see the Spotted Fritillaries from May onwards it would be wise to be mindful of this difference in appearance. The Lesser Spotted Fritillary has still not been confirmed from Corfu but it could exist there.
You can see more characters useful in the identification of the Spotted and Lesser Spotted Fritillaries on my website.