[GRID] Detect sortable headers

I would like to add custom class to headers of these columns which are sortable, to display some kind of indicator, that this particular column is sortable.

How could I possibly accomplish that? I mean detect which hdrcells allow sorting?

There is an inner collection grid.fldSort which holds sorting types

for (var i=0; i<grid.fldSort.length; i++)
if (grid.fldSort[i]!=“na”)
grid.hdr.rows[1].cells[i].className+=" customOne";