In a strong bull market in 2013, most mutual funds have performed fairly well. But as the end of the year approaches, some mutual funds resort to a sneaky trick to try to convince investors that their managers made smart moves that they actually missed out on.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, looks at the practice of window dressing, by which mutual funds buy winning stocks right before the end of a quarter so that the stocks will show up on their quarterly holdings report. Dan explains that fund managers don't want to look like they missed out on big gains, so the temptation to buy winners puts a lot of pressure on them. Dan observes that this quarter, (NASDAQ:AMZN) and FedEx (NYSE:FDX) are likely candidates for managers to use for window dressing, along with Valero Energy (NYSE:VLO) and Micron Technology (NASDAQ:MU). Dan goes through the reasons why Amazon, FedEx, Valero, and Micron have done so well, but points out that if funds haven't actually generated strong returns, investors should be suspicious and check to see just how long the fund has owned those holdings.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.