Honestly, it's rarely difficult to tell which role is struggling. The problem often comes from people refusing to adapt when the situation calls for it. As a DPS, if I notice I'm partied with an undergeared tank, I'll avoid pushing my DPS because I know I'll pull hate from them. They might be new and trying to get a handle for their job. Likewise, if the healer is getting a bit overwhelmed, I'll pop Bloodbath/Second Wind or even just tell them to focus on the tank. This works in the same if it's the DPS struggling.

Now if a person's just being plain lazy, I might let them die depending on my mood.