There are literally books written about how one goes about trying to define signs or symptoms of abuse. The problem with most of those lists is that the very same behaviors can be a sign of all kinds of things having nothing to do with sexual abuse.

Teenagers are moody enough. Their self-esteem is variable enough from one moment to the next. It's hard to identify a clear-cut sign that a person has been abused. There are very few, if any, sure signs.

It's an issue of not wanting to punish someone who is a member of your family, even if that closeness includes abuse. A lot of wives do not report spouse abuse. Lots of families do not report sex abuse because it's still Dad.

The earlier you intervene, the better chance you have of breaking that cycle.