Explain how you can determine, without measuring any angles, wether a quadrilateral is a rectangle?

The figure is a parallelogram if diagonals bisect each other
If the above is true, AND if the diagonals are congruent, THEN the figure is a rectangle.
Therefore the steps to follow are:
1. Draw the diagonals.
2. Check if the diagonals bisect each other. (true or false)
3. If 2 is true, then check if the diagonals are congruent.
4. If both 2 & 3 are true, then the given figure is a rectangle.

These are steps typically used by carpenters and surveyors to check for rectangles.