Difference between Square-tailed Drongo-Cuckoo and Drongos

The Square-tailed Drongo-Cuckoo is a cuckoo but it looks similar to many drongo species. These birds are mostly distinguished by the areas with glossy hair and those with velvet (without gloss) hair: the face, the head, the hood. In forests with thick canopy lacking light and quick sighting, this is often quite tricky.

The difference between Square-tailed Drongo-Cuckoo, and Black Drongo, Bronzed Drongo, Hair-crested Drongo in Vietnam:

1/ Square-tailed drongo-cuckoo (Surniculus lugubris)

Smaller size, 
Black and hairy hood (head and neck), 
squared tail or tail slightly notched with flared tips,
white bars undertail.

2/ Black drongo (Dicrurus macrocercus)

Bigger size,
velvet hairs (not glossy) around the base of the upper bill (upper mandible, maxilla),
smoothly black hair all over the body with some glossiness or glimmering irredescence,
forked tail (except for not fully mature).

3/ Bronzed drongo (Dicrurus aeneus) 

Smaller size, 
velvet hairs (not glossy) around the face,
glossy on head, nape, mantle and wings,
forked tail.

4/ Hair-crested drongo (Dicrurus hottentottus)

Plain black on the head, face, but shiny on the nape,
velvet hairs (not glossy) around the upper wings (or upper the mantle),
forked tail or flared tail.