Plenty of F-VF examples exist for most W/F. Most DL watermarks are fairly easy to spot; SL watermarks can be easy or difficult or somewhere in between. Yellow or orange stamps are always the most difficult. The "good ones" are rare for a reason. Generally the common ones are VERY common, so one is far more likely to run into them. With some experience, telling W/F apart becomes much easier. This may help:
http://www.stampsmarter.com/index.html