To estimate the size of an angle, try thinking in terms of counting in quarter turns, or right angles.
For example, the angle in the following diagram is acute - the line turns less than a right angle to get to the other line, so it must be between and .
The angle in the following diagram is a reflex angle. The line turns through more than two right angles, but less than three right angles, so is between and .
To be even more accurate think in terms of angles.
This angle lies somewhere between and , but seems slightly closer to , so you could estimate that it is .