Sexing Common Whitethroats needs a cautious approach since many birds (incl. some adults) are quite intermediate and variation is marked. Even experienced ringers regurlarly make mistakes, see e.g. Waldenström & Ottosson (2000).
1cy birds should generally be left unsexed, while many, but far from all, 2cy+ can be sexed according to the below:
2cy+: Males have pure ash grey feathers to crown (variably concealed by brownish tips when fresh), greyish LC, a pinkish hue over the breast (may be concealed by buffish tips when fresh) and a cleaner whitish throat. 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.