Is there a standard symbol for an inlet on building electrical plans?
I did a quick review of my symbols library including an international crossover reference and really didn't find any real de facto symbol. I suppose someone could have come up with one and included it in a legend somewhere but it certainly would not be a standard ANSI symbol. I guess you're stuck with using a specialized receptacle and identifying it as an inlet. Usually these things are identified on a riser diagram and try to show some sort of schematic description of what it really is.

Sorry I couldn't help.