How do we know WHEN to get the result from quantum computers? So I always hear that you can't disturb quantum computers because this will ruin the particles superstate.
Well, how do we know WHEN to retrieve the result from the calculation?
How can we determine when the calculation is finished, without observing it?
 A: As explained on page 7 of Deutsch's first paper on quantum computation, you can set aside one of the qubits of the computer as a flag for completion of the computation. The qubit starts with the value 0 and the computer doesn't interact with it unless the computation terminates, at which point it sets the bit to 1.
A: Even if you don't know the outcome of an operation, you know that you've applied that operation. So a quantum computer knows when to measure the same way it knows when to do anything else: it applies the operations it's told to, and measures when it reaches a 'measure' instruction.
Quantum algorithms are made up of operations. You apply them in order. Some of those operations are measurements. If the algorithm's 5th operation is a measurement, then you should measure after applying the fourth operation but before applying the sixth operation.
For example, here's a quantum circuit diagram with a measurement. It says to measure the second qubit after applying the controlled-not gate, but before applying the second Hadamard gate:

(You can re-order the measurement and second Hadamard gate without affecting the outcome, but the circuit as drawn specifies a particular order.)
