The Second Sleeve

After what seems like an eternity knitting, I have finally breached the second sleeve of my green shrug. However, this accomplishment was immediatly met with disappointment as I lost count of my rows. I had gotten out of the habit of counting during the back because I just needed to knit for 21". Now that I am back to the sleeve, I obviously want the two arms to be the same length. Although, I'm not really sure that my arms are really the same length either - so maybe I'm actually making a custom fit shrug. So my question is this: Is their an easy way to tell how many rows I've knit? I have a marker inserted where my back ended, but I have always had a hard time telling how to count rows in the fabric itself. Oh well...