You shouldn't have to ask. Generally, people who pressure you to buy have higher prices. The grades cannot be verified on a mounted diamond. Then have it mounted in the setting of your choice. Put guarantees in their proper perspective.
Some guarantees are nice but most are fluff with many exclusions.
Decide where to buy based on the central issues of quality, price and service first , then read any guarantees to see if they add anything material. Have anyone quoting you prices put them in writing with the complete grade. The following diamond grading form does a nice job of asking someone to put in writing what they are claiming verbally. Anyone who won't fill it out completely and sign it should not be trusted. Do this before you buy.