I don't think photoshop elements has that feature (color sampler tool for marking
color sample points).
Try making your threshold adjustment layer, add a new blank layer above, select the brush tool, select the Assorted Brushes presets, select one of the circle brushes
and set your foreground color to something you can see on the photo.
Go back to threshold and set the black point, switch to the blank layer and add a mark with the brush. (repeat for the white point).