Touching up Stained Trim

I've had to move a wall in our kitchen, which reqired me to rework some of
the stained crown molding. The first problem is that I can't find
replacement molding (ours is a unique profile). So, I'm having to re-use
some of the existing material. It would be nice if I knew where the molding
came from, or how it was finished, but that information was probably lost
way back in the house's ownership chain.

Long story short, I had to touch up the stain on one piece, and then
squirted it with some spray-on polyurethane. It looked good when the poly
was wet, but a day later, the section I touched up has turned milky white.
Obviously some sort of finish incompatability.

The question is, how do I figure out what finish is on the crown molding, so
I can touch it up with something compatable?

Thanks in advance.

KB