Viewing a Colleague's Activities is done from the Activities menu
Click Activities > Click the arrow to change your view > Choose All Activities
Click Owner > Click Filter By
Choose the name of the colleague(s) who's activities you'd like to view
Click Apply
You are now presented with only the Activities of the user(s) you've selected.
If you are looking for something specific you can use the sorting options. Sort by when the activity is Due or filter out only specific activities by placing a tick in the box next to the type(s) you are looking for.
In this example, Email activity type only was chosen, narrowing the list of 95 down to 5 making it mush easier to find the specific Activity in question.
In some cases, using a Chart might be helpful.
After you've filtered by Owner > Click Show Chart
Choose the Chart you'd like to use. System Charts are available for all Clients. My Charts are charts built and shared specifically for your organization.
Activities By Type is a System Chart with a pie graph. Hovering your mouse over or clicking on any section of the graph will then filter out the Activities on the right side list view
In this case, the 19 Tasks were clicked automatically sorting the list to only Tasks.