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According to Falk:

Caesar - Females: Tergite 6 with short marginal bristles at the side and 2-4 at the mid-dorsal line but with a gap in between; frons typically wider than illustris, 3rd antenal segment typically longer and aristae with more rays on underside than illustris (these characters notoriously variable). Subcostal sclerite with small black setae near apex in addition to the microscopic pubescence. Tergite 1+2 greenish and not contrasting strongly with subsequent tergites. 3rd tergite without obvious marginals.

Illustris - Female with complete row of marginal bristles on tergite 6. Subcostal sclerite with small black setae near apex in addition to the microscopic pubescence. Tergite 1+2 greenish and not contrasting strongly with subsequent tergites. 3rd tergite without obvious marginals.

Bufonivora - Usually only 2 postsutural dc. Female frons about 1/4 head width. Subcostal sclerite with microscopic pubescence only. Tergite 1+2 black, contrasting with subsequent tergites. 3rd tergite with conspicuous marginals.

I think someone very experienced with this genera could make a good educated guess from photos if they had good quality photos from a variety of angles, but that is the best anyone could do. For positive ID you need the specimen to examine genitalia and other features under the microscope.

Some of the (somewhat) easier to see distinguishing features to look for with Lucilia are:

1. The number of postsutural acrostichal and dorsocentral bristles.
2. The width of the frons (in relation to the width of the head, or an eye).
3. The length of the 3rd antennal segment (in relation to the width of the frons or its own width).
4. The number of anterodorsal bristles on the mid tibia.
5. The colour of the basicosta.