The easiest way to remember it is:
If your'e doing ribbing, whatever you did in the previous row at that stitch (knit or purl) do that again.
If you're doing seed stitch, whatever you did (knit or purl) in the previous row, do the other thing.
As for how to work out if it's a rib or not, if the pattern doesn't tell you (which would be rare, I would think) just follow what it does tell you to do on that row. After you've done that for 2 or 3 rows, you should be able to see what you've got. If it's a rib, it'll be obvious.
And if you still can't work it out, then just call the pattern writer an idiot and do it how YOU want to! :mrgreen: