The clue here is in the verb. Il est fort (He is strong) v. Ils sont forts (They are strong) Otherwise, the difference is that when next to a vowel, the 's' in Ils is carried over in a liaison.
no, is not a differnet prononciation of "il" and "ils", but for "ils", the verb will be "sont" and for "il" it will be "est"
(I am french, sorry if my english is not correct...)