There are no signs. Some people conform to stereotypes, some don't. Best bet is ask them? Try for a non judgemental/ blaming way. Like "such and such says you do coke, is that true? why, Whats it like?" Bare in mind that plenty of people take drugs and don't become homeless or prostitutes.
If you are concerned about your friend in a OH MY GOD!!! DRUGGSSSSS! Kind of way, don't worry so much. If their behaviour has become strange to you, ask them why they do certain things (change of friends/social scene, change of routine/ personal focus etc) If you are worried about their immediate health (increasingly erratic behaviour, looking ill, mental health concerns) then you need to speak to them, but again, don't blame or say things like "why do you do it to yourself" as this will stop them wanting to talk to you.
essentially, whilst under the influence, coke makes people pushy, gobby, the life of the party (in their head) and flu like symptoms the next day. Marijuana makes people think they are insightful and clever, whilst lying on the floor staring at the walls. Lots of talking, slowly about unusual subjects. Signs of use include cigarettes with no tobacco in them (but paper and filter still in place) large rolling papers, strong smell. Coke doesn't really have any signs short term, perhaps more/less money?
you shouldn't worry too much about casual drug use, if its "just at weekends etc" its no different really to alcohol. There are definite risks involved but thats the case with most things.
To be honest unless you are concerned that there is an immediate issue, I'd steer clear of getting involved, people don't like being told how to live their lives, especially by people who don't subscribe to their ideals.
HTH