Several points:
1) You seem to be asking about "value" with regard to SELLING. It should be understood that "value" is subjective. "Value" to whom, and in what context? The "value" of an item will be different if you want to sell it to a jeweler, buy it at retail, have an insurance appraisal, sell it to someone privately (or on ebay), or have it evaluated as part of your estate. Same item, different "values"... and anything may be appraised at ANY of these levels, although an insurance apraisal is the most common.
2) There is no reliable, inexpensive tester the layman can buy that will determine the nature of material (particularly precious stones) easily and accurately, let alone the "value".
3) Jewelers are not in the business of trying to cheat you, with VERY rare exceptions. Some jewelers will offer you a verbal opinion as to the nature of a particular item you have, and may even opine as to it's "value", but most will only offer a formal written appraisal, which you must pay for... and that's NOT the same as an offer to BUY. If you're looking for an offer from a jeweler, you should tell them so "up front". Remember, some jewelers DO buy from the public, but most do not.
4) As with many other things, one of the best methods of determining the best deal is COMPARISON. If you receive an offer, it's always best to go elsewhere for a second (or third) offer, THEN decide what the best deal is. By all means, check with a pawn shop, but unless you have no alternative, NEVER sell to this type of operation without a comparison.
5) If you're not sure about a jeweler, check with you local (or state) Better Business Bureu. It's easy, and it's FREE.
6) And, you would know the karat content of a piece... how, exactly? If it's stamped? I own a "14K" stamp and could use it on a paperclip if I wanted to. Would that make it gold? A jeweler can usually check. You, on the other hand, most likely cannot.