Sexing Common Whitethroats needs a cautious approach since many birds (incl. some 3cy+) are quite intermediate and variation is marked. Even experienced ringers regurlarly make mistakes, see e.g. Waldenström & Ottosson (2000).
Many, but far from all, birds can be sexed according to the below:
2cy/3cy+: Males have pure ash grey feathers to crown (often with some brownish tips, if still fresh), greyish LC and a pinkish hue over the breast. Females, on the other hand, should show no pure grey on crown, no more than little grey on LC, no pinkish (but often a creamy buffish) hue on breast and a weak buff hue on the throat.
Still, due to variation, a few birds are likely to end up in the wrong catergory since males (generally 2cy) may show a very female like plumage and, conversely, females (generally 3cy+) may attain male like characters.