As a rough estimate, whenever your opponents reach a major suit or 3NT game contract they will hold about 26 hcp.
Therefore, as a defender, you know that you and your partner have about 14 hcp between you. So, to estimate how many hcp partner has, simply subtract your hcp from 14. This rough guide will often allow you to place a missing honour or two and to conduct a successful defense.
Knowledge of your partner's approximate hcp can also help you with signalling. For example, if you hold almost all the missing hcp, there is usually very little point in giving signals, as they will probably only help declarer.