qwpDataset = new LABKEY.QueryWebPart({
    frame: 'none',
    queryName: dataset,
    renderTo: 'someDiv',
    schemaName: schemaName
});

In the button bar using the "PAGE SIZE" drop-down menu pick "Show All", then in the console:


dataregion = qwpDataset.getDataRegion();

console.log( dataregion.totalRows );
> null


For other "PAGE SIZE" drop-down numeric menu options, the value is set correctly.
I'm relying on it to see if the dataregion/QueryWebPart is empty or not (without requerying anything).
If there's an alternative way to tell, I'd like to hear about it.

So far I'll have to rely on querying the DOM for "tr.labkey-alternate-row" and "tr.labkey-row" and summing them up :(