You can tell if if their left handed or right handed very distinictive , you can also tell if someones happy or sad when rightintg and how mucn time they spent by the puncation , happy is slanted forward and sad its when its slanted backwards.

I am sure it was a double bluff , the cabbie wasnt a danger to sherlock due to sherlock knowing that john would save him perdsonallly ,  The only danger was himself , he was tempted but stopped when the shot came its a bit obvious , John who was a army doctor would know that it wasnt even safe with those pills.