As an assistant referee, what is the proper way to indicate that a goal has been scored when not immediately apparent to the center referee that the ball has crossed the goal line between the posts?

USSF answer (September 15, 2011):
We are not authorized to answer questions regarding high school soccer, but this question covers material that is universal. This guidance from the USSF publication “Guide to Procedures for Referees, Assistant Referees and Fourth Officials” should answer your question:

• If the ball briefly but fully enters the goal and is continuing to be played, raises the flag vertically to get the referee’s attention and then, after the referee stops play, puts flag straight down and follows the remaining procedures for a goal
• If the ball clearly enters the goal without returning to the field, establishes eye contact with the referee and follows the remaining procedures for a goal
• Runs a short distance up the touch line toward the halfway line to affirm that a goal has been scored
• Keeps moving to avoid confrontation if approached
• Observes the resulting player behavior and the actions in and around the penalty area
• Takes up the position for a kick-off
• Keeps players under observation at all times
• Records the goal after the trail assistant referee has recorded it