There are two main ways to view previous chat history in Trillian: scrolling back and through the dedicated history viewer. Trillian supports infinite chat history, meaning it will continue to load and render previous messages within a chat window as long as you continue to scroll the window upwards and as long as there is content to render. In addition, the dedicated chat history viewer exposes calendar-based browsing and a search option.
Page Downkeys on your keyboard. This is a keyboard-based method that accomplishes the same thing as dragging the scroll bar upwards.