The signs I've seen of heroin use...
nodding off, slower, sleepy, hard to talk to cause it's like its just not getting through... withdrawals ... lots of sleeping, not feeling well, cold flu symptoms.
Do some research and learn all you can. Knowledge helps. Also lots of addicts shoot up in places hidden, between toes, etc, so looking for tracks in the usual places (arms) doesn't always show the whole pic.
Unfortunately I know a few people with heroin problems. None of them started out with shooting up. (most started with smoking it and moved up, so the use he's admitted may be, well I only injected x times, you didn't ask about me smoking it...)
Unfortunately, I do not know of any one who has been able to use heroin in a recreation way with out it wrecking their life.
Sorry you are going through this. It is better to know than not.