Spiders will come back to reindex your site on a regular basis as long as you keep the content fresh. If the content has been the same after multiple indexings, they will come back less frequently.
There are two options. First see if you can get access to your website access logs. Here you can check to see when Googlebot, Yahoo Slurp, and MSNbot are coming to your site. Secondly, I´d invest in a better tracking program. Most tracking programs will allow you to see which "browsers" your clients are using. Just look for those bots above to see how many times and how often they are coming to your site. You might also look into Google Analytics. It is a free (as long as you are under X amount of page views a month) and might show you the information you are looking for. We personally run our own custom built tracking program that provides us with this information.