First off, a CNF (or DNF for that matter) consists of a formula, or in other terms a "well-formed formula" or "statement form", by definition. So, the expression comes as unambiguous, and thus if you have values for the variables, the function will return a definite value. In fact, the very definition of a function means that if you all the variable(s) get assigned value(s), the function will take those value(s) and return a single output. When someone indicates something like (A∨B)∧(¬B∨C∨¬D)∧(D∨¬E) as a CNF they probably mean it as an abbreviation for a fully parenthesized form (it doesn't much matter if we left-associate or right-associate ^ and V here, since those CNFs come as equivalent), or they don't quite understand the definition of a CNF.
Now, that said I think you mean to ask if an expression which you can identify as a conjunction, looking like an abbreviation for a conjunctive normal form, will necessarily come as unambiguous, or equivalently evaluate to 0 or 1. Or in other terms if such an expression will necessarily also express a function. Well, consider
(A v ¬A) ^ (B V C ^ D) ^ (C v ¬C).
Say we have B->1, C->0, D->0. Well, then ((B v C) ^ D)->((1 v 0) ^ 0)->(1^0)->0, while (1 v (0 ^ 0))->(1 v 0)->1. It follows that even though we can tell that (A v ¬A) ^ (B V C ^ D) ^ (C v ¬C) should indicate a conjunction, it simply won't come as unambiguous, or equivalently evaluate to 0 or 1. In infix notation you either need everything fully parenthesized, some form of interpretation (which an algorithm basically doesn't allow for), some rules for order of operations (and you'll need such rules to cover all possible operations), or you've gotten lucky.