Certainly additional sales will be a sign of success, but because of the way the web works -- ie, people will view a site, bookmark it, then return later (days, weeks, or even months!) to make a purchase -- immediate sales will not tell the whole story. Therefore you need to be able to track your "unique visitor count" from one day to the next.
For example, let's say you have an ad coming out in a magazine and the publication date is the first of September. On August 30th you have 100 unique visitors; on Aug 31st you have 97; then on Sept 1st it shoots up to 400; Sept 2nd it's 650; Sept 3rd it's 890, and so on until it settles back down 2 or 3 weeks later to about the same levels as late August. That of course is not an anomaly -- the spike in the stats is clearly a result of the ad.
Please note... there is a significant difference between "hits" and "unique visitors" -- if you do not understand the meanings of those terms, here is a brief [explanation].
You may already have access to some pretty good stats via your control panel at your site's hosting service, so first check to see if that's the case. If not, we have a number of tracking options (both free and fee based) at JOLAF's [exhibitor resources] page.
Some stats programs are very sophisticated and give a lot of important details, such as your unique visitor count; or the referring sites (like JOLAF) that sent the most visitors your way; or the search engine queries that people used to find you; etc.
Other programs however are simpler and only show more general numbers. The important thing is to have the ability to get dependable feedback about the performance of your site, so even simple relevant tracking is better than nothing at all.
(Note that we are NOT talking about old fashion "hit counters" -- the information they provide is close to useless -- see the [explanation] above).
Once you have the capability to view genuinely relevant statistics, then try to access that data at least once per week so you know where you stand. The trends that you see over time will tell you a great deal about what you are doing right, and, what you may still need to do if you want your website to reach its potential.
Final Note -- If you know that you are getting a good traffic flow but are not making any sales from your site, then that is an entirely different issue. This brief [ecommerce checklist] may help explain why the web orders are not happening...