How to tell if someone has a cocaine habit, is often a study of the person.
If you are familiar with the person it becomes easier.
If you are not familiar with the person then you have to look for the obvious.
Obvious signs:

• Extremely talkative
• Dilated pupils
• Restlessness
• Fast speech
• Super alert
• Lack of fatigue
• Seems to not need sleep
• Anxious
• Irritable
• Not responsible

Physical signs:

• Bloody nose
• Runny nose
• Red and/or swollen nose
• Wild eyes
• Dilated pupils
• High blood pressure
• High body temperature
• Having seizures

The physical signs of a cocaine user will vary with the person.
Often the eyes are a good indicator, they will usually appear to have wide eyes and be bloodshot and the pupils will be dilated.
Some of the subtle signs are an increase in heart rate and blood pressure causing heart irregularities.
Cocaine users will often have a heightened speech patters, so if they show signs of irregularities in their normal speech it could be a sign.
Cocaine is also a appetite suppressant so you will often see a weight loss and may very well notice that they are not hungry at meal times.
Because cocaine is basically a stimulant, users often have a hard time falling asleep and have inconsistent sleep patterns.
They have to come down from their high before they can sleep.
One of the most serious telltale signs, if you are familiar with them is the inability to fulfill normal, simple and regular duties.
One sign of this is the deterioration of personal hygiene.
Often the user will give up, or seem to forsake, family and friends in their quest for the next fix.