A sticky is "active" (it receives text in the text area or shortcuts input) when the "status image" (a little top-left triangle) is color green (arrow 1). If not... click anywhere into the sticky itself to activate it. If the sticky is not active the status image color is violet.

Another element that helps to understand "at a glance" if the sticky is active if the cursor is blinking in the text box (see arrow 2). However, this is not always true: when a sticky jumps on the screen because it was reached the alert time the text box is temporarily disabled and the cursor (2) therefore does not blink.