Actually, you can't know this until you get a lot of measurements to see the pattern. E.g. if you just put one photon through you get one dot, and you'd be hard

Actually, you can't know this until you get a lot of measurements to see the pattern. E.g. if you just put one photon through you get one dot, and you'd be hard pressed to infer anything from that. So in the end you still need a lot of measurements to detect interference. Also I bet you could mess with the interference pattern by pulsing the light source. This would be the lesser known form of the Uncertainty principle, where energy and time are unknowable.