A light meter will show you what you cant see in terms of light rays, but in general yes, windowsills do not put out nearly anywhere near the same light as overhead lights or the sun. It would depend on how directly the sun is coming in through the window I guess, but no matter, even if it were unless it were a greenhouse there would be a side of the plant that doesn't get light. None the less, indirect sunlight has been shown to not work time and time again. You might be able to grow a few leaves this way but you can never pack on the flowers and fruit that you are really wanting from this grow.