when you find out a friend or co-worker self-injures it's very important to refrain from being judgmental. people deal with their inner turmoil in different ways, and self-injury is only one of the many ways.
support your fellow human beings and offer to listen should they need an ear. recommend them to someone that may be able to help. it's very important that you do not try to take matters into your own hands. always let them know you will support them.

if you suspect that someone you know self injures, it's important to recognise some warning signs

note:most of these warning signs are for episodic self-injurers, though they may and probably will apply to all kinds of s.i.'ers.

SUDDEN CHANGE IN MOOD: you may notice periods in which a self-injurer's mood may change. they may act more defencive, irritable, unmotivated, happy, etc. or completely out of character

ELUSIVENESS: sometimes the self-injurer may become more elusive or evasive during episode. they may seem as though they are hiding something, sometimes not realising it themselves. they may mention something suggesting their actions without thinking about it but be quick to avoid answering questions or changing the subject should the issue be questioned further. sometimes they made avoid questions altogether.

UNEXPLAINED MARKS OR BANDAGES: when asking someone where they got that bruise or why they're wearing a bandage and your answer is vague it could signal that they have self-injured. some self-injurers will fabricate a story or use an unrelated event to explain what happened.

WEARING LONG SLEEVES IN HOT WEATHER: or long pants when they habitually wear shorts or miniskirts could indicate that they are hiding marks.

ISOLATION: sometimes the self-injurer may become more reserved in themselves (when forced into public situations) or rarely leave home. they may also seem apathetic in things they usually enjoy.

PUSHING FOR ATTENTION: e.g. feeling the need for excess validation or just the need for to someone to be around. they may not explain why they need someone around.

MOOD SWINGS: from happy to sad to isolated to angry or any other swings in attitude within a short period of time usally happens before and/or after the self-injurer hurts themselves.