How to check for Unassigned Mat

A few tips

First, to clean up terminology, open this page in the Mxs-help: "Collections > Collection Types > MaterialLibrary Values". As you see, their is 3 system globals described at the begining - currentMaterialLibrary, sceneMaterials, meditMaterials. They a 3 independent Collections. All of them hold material instances, but sceneMaterials hold only all materials applied on the scene. currentMaterialLibrary you can access w/o moving it contents to the MEditor, and if the mat's are in the MEditor anyway then use meditMaterials collection. For example: myMat = meditMaterials[1] -- will assign the mat from 1st ME slot to a variable.

Next, as you say you will check a selection of objects, then searching in the sceneMaterials is not the case here. So you need to 'scan' your the .material property of your selected objects. Something like:
for obj in selection do if obj.mat != undefined or obj.mat.name != myMat.name do obj.mat = myMat;
but... this example code compare only the names of the materials. So this is the next question - what you want to compare.

And finally, why you ever need to check the current state of obj's mat's? If the mat is already applied, adding it again will not change anything, nor yet will slow down the proces, if you do this of course in 1 mapped call, i.e. for ex. just:
selection.material = meditMaterials[1]

I hope this helps.
Cheers!