speed up check in SymmetricFunctions

src/sage/combinat/sf/monomial.py

@@ -171,11 +171,11 @@ def from_polynomial(self, f, check=True):
             m[1, 1] + 2*m[2, 1] + 3*m[3]
         """
         assert self.base_ring() == f.base_ring()
-        out = self.sum_of_terms((Partition(e), c)
-                                for (e,c) in six.iteritems(f.dict())
-                                if tuple(sorted(e)) == tuple(reversed(e)))
-        if check and out.expand(f.parent().ngens(),f.parent().variable_names()) != f:
+        if check and not f.is_symmetric():
             raise ValueError("%s is not a symmetric polynomial"%f)
+        out = self.sum_of_terms((Partition(e), c)
+                                for (e,c) in f.dict().items()
+                                if all(e[i+1] <= e[i] for i in range(len(e) - 1)))
         return out
 
     def from_polynomial_exp(self, p):