Read the articles I linked to or do some research on the terms. Those people are much more subtle than your run of the mill troll. They like to prey on the insecure ones. The ones that don't have enough knowledge to understand the basic principles of why proprietary software is less desirable than FOSS. They will often use sentences like:

"I love Linux. But, this new Microsoft gizmo is amazing."

Of course, not everyone using such reasoning is necessarily an astroturfer. But, when ever I see such arguments I certainly become suspicious. With a few good questions you can usually make them either show their true colors or go away.

Moles are even harder to spot. They will even volunteer to help and do good work for a while. Eventually, though, they will start to stir things in a way that benefits their true, proprietary, agenda.