You can find many guidelines like these in the "Smalltalk with Style" book.
For instance on page 3 it says "Upper and lowercase letters also distinguish variable scope. Variables beginning with an upper case letter (globals, classes, class variables, and pool dictionaries) are global to all methods within the definition scope of the variable. Method parameters, temporary variables, and instance variables begin with a lower case letter. By convention, class and instance method names begin with a lower case letter".
You can find this and other Smalltalk books here:
http://stephane.ducasse.free.fr/FreeBooks.html