One way that I use is to place a stitch marker on one of the rows when I start and then take it off when I am done and start on the next row. This way, if you write down which row gets the stitch marker and which one doesn't, then you will know just by looking for the marker. The other way is to keep a tally of rows. Make a column for the 80 knit and a column for the 80 purl row and then just put a tick in the column when you complete a row.
Those are 2 ways I keep track.