I don't work with palmettos, but if there is a visual marker for a het palmetto then I've never noticed it. Most of the het I've seen just look normal, or like whatever other morph/s they may have. I did find an article that says otherwise though.
Google is giving me mixed messages. iansvivarium and cornsnake.net are saying it's an inc-dom while cornguide.com and morph market list it as recessive. Sounds like someone needs to get their act together.