Follow any of these links for more information on how you can do it too:

I don't always specifically mention that I'm being paid for a post, but you can always tell. Starting this month, I tag all paid posts with a date. For instance, all paid posts this month will be tagged with '01/08' - next month '02/08' - and so on, so check the labels. It's also a good way for me to easily see how many paid posts I do every month.