How to show “if” condition on a sequence diagram?

I was wondering, how can one represent “if” statement on a sequence diagram?

if (somethingShouldBeDone) {
       // Do it
} else {
       // Do something else
}

Can it be represented at all? The thing is … in my code, fair amount of conditions are checked to determine a variety of actions. If i am going to show the actions, I’d like to explicitly state that actions are caused by particular events.

If possible create an image representation of a solution.