Is it easy for trained dermatologists to suspect melanoma? do they miss it often?

Usually: Trained derms generally don't miss melanomas.

Borderline lesions+: Sometimes some are so benign looking naevi it's possible to miss diagnosis .After biopsy pathological exmn especially small lesions may be reported as negative for malignancy . Despite removal a nevus/mole suspected for malignancy , with reasonable cleara.Margin during biopsy may recur at the same place or can present in another skin area as metastases or lymphnodes close to primary after years.

Part of training: A big part of a dermatologist's training is the visual recognition of skin conditions including melanoma. Anybody can miss things but dermatologists as a group are less likely to miss skin cancers than other doctors.

Possible: A well trained dermatologist is able to detect lesions that are suspicious for melanoma.