[...]
I had worked out a rule to distinguish between "c'est" and "il est" for the same italian friends :
You are about to choose between "il", "elle", and "ce".
1) Does this pronoun refer to something that can be named?
If not, then choose "ce".
Example : "C'est la vie", "C'est comme ça".
2) Otherwise :
2 a) If what comes after "est" is an noun attribute, always use "c'est".
(Voici Paul, c'est mon ami : (Mon ami) is a nominal group)
This rule is very powerful. The cases given by Tesley (profession, etc.) are only a very small subset of the possibilities.
2 b) If what comes after "est" is an adjective attribute, always use "il" or "elle".
(voici ma bicyclette, elle est rouge)
2 c) If what comes after "est" is not an attribute, always use "il" or "elle".
(Où est le livre? Il est dans la cuisine).
Exception to the rule 2 b: If "il" or "elle" refers to a (relatively) big place, you may use "c'est" instead. 5(La France, c'est grand).
The bigger the place, the more recommended the substitution.
("J'ai vu ta chambre, c'est beau" (a room in not a very big place, you can also say : "J'a vu ta chambre, elle est belle".)
Note about case 1) : This case will not occur very often if you want to translate the English "it is". In situations like in case 1, You will most often rather say "that is" in English, and the phrase "that is" always translates as "c'est", never "il est" or "elle est".
N'hésitez pas à mettre cette règle à l'épreuve : Elle a largement besoin d'être améliorée.