To determine conditional probability using a Venn diagram, you need to identify the overlapping region of the events and divide it by the area corresponding to the given event.

A Venn diagram is a visual representation that illustrates different events as circles within a universal set. To find the conditional probability of event $A$ given event $B$, denoted as $P(A|B)$, you should concentrate on the areas represented by these circles.

First, locate the overlapping region between circles $A$ and $B$. This overlapping area represents the intersection of events $A$ and $B$, denoted as $A \cap B$. The intersection contains the outcomes that are common to both events.

Next, determine the area of the circle representing event $B$. This area encompasses all possible outcomes where event $B$ occurs. The conditional probability $P(A|B)$ is then calculated by dividing the area of the intersection $(A \cap B)$ by the area of event $B$. Mathematically, this is expressed as:

For example, in a Venn diagram where the area of $A \cap B$ is $0.2$ and the area of $B$ is $0.5$, the conditional probability can be calculated as follows:

This result indicates that, given event $B$ has occurred, there is a $40\%$ chance that event $A$ will also occur. Utilizing Venn diagrams not only aids in visualizing the relationships between different events but also simplifies the calculation of conditional probabilities.