There isn't really any particular standard that I'm aware of for city point symbology. It's generally a dot or dot within a dot for a city, with the size often relating to the size or importance of the city. Capitals often have a star, star on a dot, dot on a star, or something like that.

There are plenty of other options like varying the label. A double underline for a national capital and a single underline for a regional capital for instance.

Use whatever provides the clearest and easiest to read result for your map and the particular information you want to convey with it.