No, a refill that fits is no proof of authenticity. The refills themselves are now being counterfeited too, and the bodies of the copies are made to such close tolerances that genuine refills fit almost any fake. The old rule of thumb that only a genuine body would take the original refill no longer holds. The same goes for paperwork: a completed service booklet with a dealer's stamp is no reliable sign either, since even the stamps have been copied.
In the end only the source of supply offers complete certainty. Buy from a Montblanc boutique or an authorised specialist dealer and you can rely on the pen being genuine. Any other purchase carries a residual risk, which can only be reduced by the seller's standing, a verifiable selling history and an inspection of the pen in the hand. In case of doubt, Montblanc will examine a pen for a fee and issue a certificate of authenticity.