How do I know when to use SOHCAHTOA and when to use the sine or cosine rule?

First, check whether the triangle in the question is a right angle. If it is then SOHCAHTOA is used every time.

If not, count how many sides we know the length of and how many angles we know the value of. Then check whether we need to calcualte a length or an angle. 

The sine rule is then used in the following cases: 2 angles known + 1 length known + 1 unknown length or 2 lengths known + 1 angle known + 1 unknown angle

The cosine rule is used in the following cases: 3 lengths known + 1 unknown angle or 2 lengths known + 1 angle known + 1 unknown length