A predicate is the object of a sentence: the thing that the verb acts on. For example, in the sentence "He kicks the ball," the predicate is the ball, since it is the thing being kicked. On the other hand, the "subject" is "He," who is the person doing the action. In this case, where we have the sentence "Is it yours?", the subject is "it" and the predicate is "yours", which is a possessive adjective. According to moxiemeg, when the possessive adjective is the predicate, you do not use the article. Or rather, you do not need to use the article.
I think both are correct. Here it is as in English: is it (è) yours (tuo)?, but normally you put the article before, unless it is family singular. (mia madre, tuo padre, suo fratello, tua nonna etc. BUT... when it is plural there is always an article: le mie sorelle, i tuoi fratelli le sue nonne etc.)
In Italian, the only way to make something a question is to add a question mark in writing, or add an inquisitive inflection on to the end when speaking. È tuo (It is yours). È tuo? (Is it yours?) In English, when forming questions, we swap the 'it' and the 'is' around, but not so for Italian.
From what I have been reading, the article "the" (e.g. il, la) is used in front of the noun with the possessive squeezed in between. For example, il tuo gatto beve acqua i.e. "your cat drinks water." However, where the possessive goes can change based on what you are trying to say. For example, if you wanted to say, "The cat is yours," the possessive would no longer go in between the article and noun e.g. il gatto è tuo. If you no longer have a noun with your possessive like in the case above, you no longer have an article either. So, in my example above "the" and "cat" are dropped, and "it is yours" is simply "è tuo" NOT " è il tuo."
Is this a good rule of thumb?
If you mean the use of the two different kinds of accent (grave = è ò; acuto = é ó) in Italian, they sound slightly different. If you write by hand you could mark both accents in the same way (although it's considered a little rough), but if you are typing using a phone or a computer you should use the appropriate one.
If you mean the use of the two different accent in this case, the third singular person of the present indicative of the verb "essere" is always "è", with the grave accent, capitalized in "È" when it's used at the beginning of a sentence.