Most all of Hull's Brown Drip, House and Garden, and Crestone pieces have "Hull USA" on the bottom, but not all.  Their most common Salt and Pepper sets have "hull ovenproof USA" on the bottom, but the mushroom Salt and Peppers do not.  Their individual leaf dishes in both the brown drip and the rainbow counterparts may or may not have the "hull" logo, but will say "ovenproof USA".  Familiarize yourself with the lettering and the logos "hull ovenproof USA" or "HP & Co."  The "USA" should be script letters with the "S" as a key reference point. Keep in mind that a lot of genuine Hull is unmarked and decide for yourself if you're willing to take a chance.  Be prepared to bid high on the truly rare pieces, chances are that you will not get them for a bargain.  Many sellers list items as Hull that are not...as though if it is brown drip, it must be Hull.  A good case in point are the vinegar and oil cruets that are jug shaped with handles.  The letters "V" and "O" do not match even remotely the script letters of the true Hull Vinegar, Oil and Cheese set, the Salt and Pepper sets, or the Cannister Set.  The bottoms of them are flat and unglazed.  However, the coloring of these pieces is extremely similar to the Hull pieces.  There were several manufacturers during the time that Hull operated that produced very similar items.  There are at least 2 very good books showing excellent illustrations of most of the brown drip pieces published by Brenda Roberts.  These are excellent resources in comparing items up for bid.  Not all of Hull's products in the Brown Drip line are listed, but the vast majority are.  Also, be aware that an F plus a number, annotates a planter that was manufactured for florists or home markets during that era.  Many sellers represent, knowingly or not, that these are unique serving dishes.  Their value should be determined by you, the buyer, but do not be mislead into thinking you are bidding on a rare piece of the House and Garden line.  A lot of the pieces also have a B plus a number.  These pieces are also planters.  You must also consider the condition of the piece before deciding its desirability in your collection.  Utensil marks are very common in good pieces, but look closely at the pictures and/or ask the seller questions to determine if an offering has a place in your home.  Chips and cracks detract from the appearance and utility of pieces, but an inobtrusive chip on the bottom may or may not be important to an individual buyer, particularly if the item is very rare.  Too many utensil marks make the finish very dull or remove it completely, especially in plates.  Keep in mind, also, that manufacturers flaws are common in Hull. While these might detract from the appearance, unless it is a major blemish, it does not detract much from the value. Lastly, if an item is unique and you've never seen one like it, ask the seller what markings are on the bottom.  I've found 3 items this way...and they're treasured in my display.