If you are talking about switching to your currently running apps/closing them, then yes hold the back button. But if you are talking about a task manager which we have in our desktop pc, or android phones, then no there are no such apps like that in WP.
It wouldn't tell you much. You'd see the exact same thing as you already see in the task switcher, with all of your open apps using 0% CPU time, except the task manager itself, which would be the only app actually running.
What I would appreciate is a performance monitor with the ability to log that information over time, but that isn't the same thing as a task manager.
Yeah, this is WP, not Android, your resources aren't exactly hogged up. Task managers and antivirus are on of the reasons I switched to WP from Android. I got tired of having to troubleshoot my phone and stuff. It's a phone, not a computer.