Almost for sure it is a 90 degree. 60 degree is very deep and usually used only on custom work. Find some tool with a sharp 90 degree corner (steel rules often fit this description) and see how that corner fits down into the existing checkering; it should fit flat/flush to the checker channel walls. Or, file up a scrap of hard, dense, close grain wood to a 90 degree and to a 60 degree (test against a 30-60-90 plastic triangle or use the triangle) edge and see which one fits.