How to find out if Restricted Area is "hot" before taking off?
Restricted areas that have scheduled times will be noted in the (sectional) chart margins. In this example you'll find this at the bottom of the Washington sectional chart:

Ones without regularly scheduled hot times will say something like 48 hrs in advance or other times by NOTAM.

As you've noted though restricted areas with posted hours may not always be "hot" during those hours - sometimes you get lucky. The best way to check if you're going to be lucky prior to departure is to ask Flight Service (1-800-WX-BRIEF): The briefers have access to the same data as ATC, and will know definitively if a restricted area is hot (at least as of the time you call them).  

Obviously you should still call the controlling agency when airborne to verify the status of any special use airspace prior to entering it (briefers can make mistakes, and some restricted areas may be activated on short notice which means they may go hot while you're enroute).