How can I tell from the location of flow separation if an airfoil is stalled?
The boundary layer separates usually at somewhere around 10-15% of chord near stall AOA (which is where you place vortex generators when using them to delay stall AOA).  Separation will start at the trailing edge and move forward.  If it was me I'd tuft the entire lower surface from just behind the LE, maybe 10% so you can really see everything that's going on.  Doesn't hurt to put too many on.