Who do you have looking at it?!
It is pretty tough to confuse cedar with pt. Cedar is much lighter in
weight and softer. The grain is finer and less prominent. If you cut the
wood,or sand it, cedar will be reddish; pt can be almost anything, but it
won't be reddish. (Freshly cut cedar seems to have a "cedar" smell, but
that could just be my imagination.)
Anyhow, what are you staining it? A solid will look the same on anything.
Semi will probably look different. Last year I stained a deck that had pt
deck and cedar posts. I urged them to go with solid, but they wanted to see
some grain. It looked like crap.