On the page for an individual item on the custom decor catalog, it lists which ages/poses the decor works for. However, once you've bought it and you're looking at it in your hoard, there's no way to tell. All you can do is try putting it on a lion and see if it works.

SOMETIMES when actively decorating a lion, the ones that don't work will be grayed out - but that doesn't seem to work consistently. It also doesn't help you decide which decors to dig up, when you have hundreds in your hoard.

I propose adding a line of text to the description of a custom decor that includes which lion ages/poses it works for.