If you are suspicious that your stats don't coincide with the actual activity of your page, it's easy to detect.  Add page counters (visible or not) to your prime pages, the number of page views per day should coincide with the number of banner views shown in your stats.  Some stats are off by a 24 hour period, make adjustments if this is the case.  If there's a discrepancy between your counter stats and affiliate stats - especially if your affiliate stats are well below the counter stats - you might not be receiving the credits due to you.

Your stats should include such information as number of views, click-thru's or hits, number of leads generated, total sales generated, commission due to you, hour of day link was accessed, etc.  Learn to use your stats to determine which merchant sponsors are generating interest and which are dead wood and should be dropped.  Give each merchant sponsor the benefit of the doubt, if after 10,000 views you have not generated a single hit, dump that program and move on.

Good statistical information will also keep a running total of your earned commissions, and should indicate immediately when your payment has been issued and in what amount.  If you have to guess at any of the above information, you might not be getting the credits that are owed to you (a common complaint) and your merchant sponsors are receiving free advertising without compensating you for the link.