some client asking many question about his site ranking, traffic and visitors flow. my point of view Good SEO Expert can not give any confirmation about this question. what you give answer about this question.
Actually there is now accurate conformation ,but you can analyse your increment ,like before after stage of your website or raking etc..The conformation can not be giving because of the huge web based business net ,one across the other. wait for more better answers .. Regards, Ahmed
example like you want to sell ads spot. they will demand you to proof your traffic/visitor . usally people will using Google analytic / Sitemeter(i just can't trust this) / Aw stats or maybe judging from Alexa Rank.
You can tell the client that you can increase the traffic, but if they have an E-commerce site, there is no guarantee that it would generate sales....