How about

let b:isLoc = len(getloclist(0)) > 0 ? 1 : 0

inside of a qf filetype window. See :h getloclist() that explains

For a location list window, the displayed location list is returned. 

All credit to romainl at 
https://github.com/romainl/dotvim/blob/44e87b3fbb829145a23bd5b2be1807cc958308e5/bundle/qf/after/ftplugin/qf.vim

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