It does tell you when. This is taken from the rules:
Big Foot, the Loch Ness Monster?there are all kinds of legends. And now PointClick has one of its own. At the bottom of every Category and Sub-category page you'll see a legend with . These symbols will be placed next to every Merchant site listing in the Network to indicate the status of each site. The indicates a site where you can use money from your PointClick account to make purchases and receive reimbursement. And the indicates a site that you'll get paid for visiting.
The hundreds of sites in the Network are always changing status. PointClick receives a large amount of traffic and our Merchants have an amount allocated to spend with PointClick to pay you to visit. Therefore, we must monitor how much traffic we send to each of our Merchant's sites. We do this by setting a "cap" on how many visitors we can send each day. Once a site reaches its cap, it is removed from the Category list until the next day.
Now, however, we will be listing the site beyond its cap. During this time, you can visit the site and still take advantage of the additional 10% discount on any purchase. But, you will not receive payment for visiting the site. Only when listed sites have the symbol next to them will you receive payment for visiting the site.
To sum it up, you only receive payment when there is a symbol next to the merchant. This is only done through the Merchant list, and only for a brief period of time during the day.