If you're concerned about someone you know, you probably have a good reason to be!
If you're not sure what's going on, ask. If you're afraid you'll be meddling, that your concerns might be unfounded, or that your friend, family member, or co-worker will be angry or offended, take the chance anyway.
He or she might deny anything is wrong at first -- abuse is a hard thing to admit and talk about.
But if you ask...
At the very least, your friend will know you care.
At most, you might save a life.