Yes this can be a good sign , but some smart dealers use lidocaine (sedative used by dentist) to get this response. In the movies you always see a blue indicator that is added to some coke , if its good it will be very blue if not only faintly. This is much like the EZ test.
Good coke (80 % and up) and believe me most people say that had 90 % pure but it was actually only about 50 % ! has a very distinct smell and taste to it , the stronger the taste the stronger the coke.
Is there any better way to tell ?