Estimating angles against convenient verticals is a useful way of checking that the direction of lines within the pose are accurate. Sometimes existing features - a doorway behind the model, and the edge of the paper - provide this reference. An alternative method, handy for smaller detail within the page, is using two pencils as a sort of protractor. This is an excellent way of minimising error and ensuring a correctly proportioned figure.
Hold them both in the one hand as shown in the example, arm outstretched, such that one pencil is vertical. Use a doorframe or corner to check if need be. Viewing the model behind the pencils, move the second pencil so it is aligned with whatever bodypart needs to be located. Then, being careful not to move the pencils in relation to each other, line them up against your drawing, extending an imaginary line from the angled pencil to draw the required line. This method is particularly useful for correct alignment of the limbs. Of course, you can also use it to check the size of non-vertical angles - such as a bent leg.
If you find this method useful, a handy measuring tool can be constructed by using a split pin to hinge two strips of strong card togther