Personally, I don't think row gauge is all that important ... especially for something like a scarf. But if you're interested in knowing how to do it, you would need to know how many rows you had knit and how long your scarf was so far.
For example, say you measure your work and so far your scarf is 4 inches long and you know that you have knit 20 rows. You would divide the number of rows by the number of inches to get the number of rows per inch .... 20/4 = 5, so your gauge would be 5 rows per inch.
This can help you calculate how many rows you'll need to knit to get your piece to be the length you want. For example, if you want a 24 inch scarf and your row gauge is 5 rows per inch you would multiply 24 x 5 = 120 rows. This isn't really necessary though ... usually most people would just keep measuring their scarf occasionally and stop knitting once it was the desired length.