To estimate the size of an angle, first think about which quadrant (quarter) of a complete turn it is in.

For example, this angle is acute, so it must be between 0° and 90°.

This angle is reflex, so it lies between 180° and 270°:

To be even more accurate think in terms of 45° angles:

This angle lies somewhere between 90° and 135°, but seems slightly closer to 135°, so you could estimate that it is 120° (or something similar).