How can you tell if someone is using heroin?

Last year around the end of July, I was awoken by a phone call from a friend of mine to let me know a mutual friend of ours had passed away that morning from a heroin overdose. I was completely shocked and dumbfounded because as this was a close friend of mine, I had no clue he was addicted to heroin let alone even using any drugs. After his death, his family and us friends just could not wrap our mind around it because there were no signs . Had there been, I would have tried to help as much as possible. He hid it very well and because of that we are now left wondering if there were signs we ignored or should have noticed. Has anyone else had anyone close to them hide their use so well before it was too late? How did the family and friends react and grieve?