Usually the first big indicator is that the game doesn't crash when you start the game or a progress, but besides that there are some things that should be present when the mod has been applied. The main menu will have a special graphic to indicate that the mod is installed, the options menu will list DickGirls as an optional sex, and the starting setup for a custom game will have new races and body part sizes, though you would need to know the normal lists to notice those. The presence of these things indicate that the mod was likely installed correctly, but modding can be complicated and it is entirely possible to add additional mods that conflict and screw up portions of the game.