markerclusterer check if marker is in cluster

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I have a web based map that is using the jquery-ui-map and markerclusterer plugin to make a google map.

I filter out which markers should be shown or not then update the map.

I need to create a list of unclustered markers and so to this end need a way to check clusters against markers and find out which are not clustered.

Is there any techniques to do this?

I have tried to cycle through clusters and manually check the markers against clusters but get an error telling me the clusters property var_clusterer.clusters_ is not defined.