Thin nacre will appear, well, thin! You may be able to discern the bead nucleus just under the surface of the pearl, and it may flash at you under close observation and strong lighting. This phenomenon is called ‘blinking’. The photo below shows a good example of blinking.
You may observe blinking typically only in Akoya pearls, as these pearls have reduced culturing times of about 12 months, so the pearl doesn’t acquire as much nacre around its bead nucleus than say, a Tahitian pearl which cultivates for 2 years, or the majority of Freshwater pearls which are solid-nacre with no bead nucleus. Blinking also occurs more frequently in smaller Akoya pearls in the 6.0-6.5 mm range.