With much difficultly apparently:)

What follows information culled from "The Fern Growers Manual" Hoshizaki. Hope it helps.

Young plants of the N. Exalata are difficult to distinguish from those of N. Cordifolia. In N.extalta the scales on the upper surfaces of the rachises are uniform in color or if darker in the center then also with irregular darker spots on the rays of the scales.

On young plants of N. Cordifolia these scales are usually whitish with a dark center sharply marked. The fronds are also narrower than those of N.Exalta.

Thanks

I bought a plant that was labelled blechnum discolor but having seen other examples since I am convinced it is actually Nephrolepsis. On the upside the cresting on the fronds is very pretty.