No your right it's not always reliable. but is with some as some species the males egg spot is really bright.
To vent you need to net the fish and flip it upside down, you will see two vents the rear is the reproduction organ and the front is the anus I believe.
A male will have 2 vents the same size where as a female will have one larger than the other.
Picture of a female from Google so you can see what I mean.
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