The radiator reservoir on most vehicles is the plastic bottle that holds extra antifreeze. Most all cars have on the side of the "reservoir bottle", small fluid level markings, some will have the word "max or min"; other markings will be small lines, and some may the words cold and hot engraved in the plastic..It can be a little difficult to sometimes see the "plastic engraved fluid level markings, and because of the color of the resovoir bottle, take your time and look closely and on all sides and top areas of the reservoir bottle, ( and check for cracks/leaks on the bottle, remember it is plastic). You always want the level of antifreeze in the bottle to be where the manufacturer recommends ( look in your "little" glove box and take out the owners manual, and look up cooling system, and most manuals will show you a nice little picture. Now if you have no book, ask a more "knowledagable" car person, they should be able to show and tell you what the marking on the bottle are within seconds, its just that easy. If you dont know a knowledgable car person the "local parts store employees will assist you, and some local repair shops will show you the correct fluid levels. Most manufacturers recommend the MAX/COLD/ highest marking on the bottle as being adequate for your vehicle not to run HOT.