In general terms fish that are pregnant or are carrying eggs become what we call gravid and have a full abdomen especially around the vent or anal regions. But it's often not that easy to tell.
As you may know all fish breed in different ways. Livebearers like Guppies and Platies do become pregnant and carry their young internally. Most other fish you'll encounter as a beginner are egg layers and they either deposit eggs on the bottom of the tank, in clumps of moss, or on plants and glass around the tank. The easiest fish to breed in the tank are livebearers generally because they do it themselves if they're happy in their environment. Next there are easy egg layers like cory's and some barbs which can be encouraged to spawn in the tank with gentle persuasion.
If you want specific information about breeding a different species there's plenty of people on here with experience about breeding many species.
Excellent reply mate. I was assuming it was livebearers we were talking about, but just realised, no actual species has been named.
What fish are we talking about here?
Emma