I use my straight needle plate when I am quilting, love it, helps a ton! I have used it for piecing, too. My husband put a small triangle of a bright royal blue electrical tape (I think it is) on the upper right corner of it for me. I always know that I have it on my machine, so therefore I don't change my stitch or move my needle and learn the hard way.
You could mark it with anything really.
I use mine.