The only way to be 100% sure is to test breed with a visual pied. There are some signs which suggest light pied, if one flight or tail feather is pied (solid yellow or white) this is likely light pied, also if a male moults in his male yellow mask he is said to only be split.
I have always had this fantasy of discovering a cockatiel that looked completely normal but was full pied. All the chicks this bird would produce when paired with a pied would be full pied... but that is just a fantasy. I call it a reverse clear pied, I don't know if they exist... but if they do it would be very difficult to identify them since test breeding many clutches for years is about the only way to know. And if somebody did claim to find one that would always be disputed since the bird could just be split. Imagine that this strain of the pied mutation could exist right under our noses and we would have no idea. That's a fun thought experiment!
I suppose it's possible! Maybe the bird would have one tiny pied feather under its wing, or something. Interesting to think about. I don't breed so I never bothered to educate myself much about genetics...but is definitely interesting.