Remember that old "American Express" slogan? Well, if you buy antiques ... and especially jewelry ... there is something you should NEVER be without. And that's the trusty LOUPE. No matter how good your eyesight is, you can't rely on the naked eye to see tiny flaws or read almost invisible hallmarks in small items.
Want to make sure you don't forget to keep one with you at all times? Put it on your keychain, like I do. You can get them fairly cheap on ebay. But make sure you get one that has 10X magnification. The ones that have higher magnification are (a) not necessary, and (b) not as easy to focus with. Diamond dealers use only 10X magnification loupes, so if it's good enough for them ...
If you think a loupe is too bulky to carry around (the standard size is usually about 1"), you can find smaller ones, but they are more expensive. A standard loupe is very cheap, and shouldn't cost more than a few dollars. There are loupes with built-in flashlights, which are very good too.
How to use the loupe:
First, find your "dominant" eye. If you're right-handed, it will usually be your right eye. If you're left-handed, your left eye (but that's not a steadfast rule ... you have to try it with both eyes and see which side is more comfortable). When you use a loupe, contrary to what you may have thought, you do not close the other eye. Keep both eyes open, even though you look through the loupe with one eye. Hold the loupe close to your face and very close to the item you're examining. You'll get a much clearer image that way.
Always keep the loupe closed when not in use, so that the glass doesn't scratch.