I'm relatively new to knitting and have just started knitting a scarf alternating purl and knit rows. The other day, though, I had to stop knitting mid-row and when I came back to it today, I couldn't remember whether it was a purled or knitted row!

Is there any way I can tell the difference?

Thank you,
Mackenzie

Your working yarn will be attached to the stitches on your right hand needle, provided you are right handed. If your working yarn is at the front, it was a purl row. If it's at the back, it was a knit row.
Alternately, you can hold the piece ready to start again and take a good look at it. If there are a bunch of little Vs, you're knitting. If there are a bunch of little bumps instead, you're purling.