An approximate and synthetic answer is that G.roulei has white lips, a pink or orange dot behind the eye followed by a line divided into 2 parts, it has two clear lines at the base of the dorsal fin while G.niger does not have white lips, it does not have a pink or orange dot behind the eye and behind the eye has a line divided into 3 parts, it has three white lines at the base of the dorsal fin. In this case, the pattern is also different but in the nocturnal livery it can be very similar. You will can to read a more complete answer shortly on the web, with Julien Renoult, Patrick Louisy and Marcelo Kovacic we have written a guide in English of 262 pages that will soon (I suppose before the end of June) be freely available on the web and explains everything in a very complete way. In the first version there will be only the genus Gobius and Thorogobius.