How to see what platform game is running on?

Do you mean something like this?

This is an extract of the ShooterGame. I guess you can use this if statement in your normal class too:

if ((Target.Platform == UnrealTargetPlatform.Win32) || (Target.Platform == UnrealTargetPlatform.Win64) || (Target.Platform == UnrealTargetPlatform.Linux) || (Target.Platform == UnrealTargetPlatform.Mac))
		{
			if (UEBuildConfiguration.bCompileSteamOSS == true)
			{
				DynamicallyLoadedModuleNames.Add("OnlineSubsystemSteam");
			}

			DynamicallyLoadedModuleNames.Add("OnlineSubsystemNull");
		}
		else if (Target.Platform == UnrealTargetPlatform.PS4)
		{
			DynamicallyLoadedModuleNames.Add("OnlineSubsystemPS4");
		}
        else if (Target.Platform == UnrealTargetPlatform.XboxOne)
        {
            DynamicallyLoadedModuleNames.Add("OnlineSubsystemLive");
        }

I just took the whole part. Only the ifs are insteresting for your though.