Wouldn't you want to know that too? I suspect that Any load that would bend the roof would require the owner to take action. I'd actually thought of doing this myself.. placing a steel rod in the center beam and having it protrude the plaster flush with the ceiling. If the roof sinks it becomes unsightly and I act. I might even put some sort of lever in line so that a half inch deflection would show as a one inch protrusion.
Which could press a button which would ignite a fuse that would drop dog food into a bowl which would then drop a bowling ball at the top of the stairs which after bouncing down the stairs, runs over the dogs tail causing him (or her) to howel and alert everyone to the load on the roof...
And you wanted to use lasers...
Certainly an interesting way to determine deflection but it requires someone to monitor it.