"venting" is the only reliable means to sex angels which are not breeding .. basically, you examine the proportionate size of the two openings near the anal fin. The opening closest to the anal fin is the vent ... in males the vent is roughly the same size as the anus and in females the vent looks larger than the anus. also, and in general, males have pointed genital papillae to facilitate the deposition of sperm over the eggs, and the female ovipositors are broad and flattened to facilitate egg laying.
naturally there are species specific variations to this rule and you need to look at a lot of fish to become 'conversant' in the "art".