Often you can’t. There are times however that you can tell she’s probably not faking it. Women might fake one or two orgasms but not over three. If she turns bright pink it’s probably real. If there is a change in wetness it’s probably real. If she starts singing afterward it’s probably real. If she wants to keep on going after your time is up it’s probably real. If she forgets to ask for the money it’s probably real.