  We consider the problem of determining the presence of genuine multipartite
entanglement through the violation of Mermin's Bell-type inequality (MI).
Though the violation of MI cannot certify the presence of genuine nonlocality,
but can certify genuine tripartite entanglement whenever the violation is
strictly greater than $2\sqrt{2}$. Here we show that MI suffices as genuine
entanglement witness even when its value is $2\sqrt{2}$ if at least two of the
local marginal distributions are not completely random provided the local
Hilbert space dimension of at least one of the sub-systems is two. Thus local
marginals suffice as semi-device independent genuine entanglement witness. This
is intriguing in a sense, as the local properties of a composite system can
help to identify its global property. Furthermore, analyzing another quantity
constructed from Mermin polynomials we show that genuine entanglement
certification task for the correlations with MI violation equal to $2\sqrt{2}$
can actually be made fully device independent.
