Unless the manufacturer of the device has released that information (or is willing to respond to your inquiries), or the device supports third party firmware (thus opening the door to discovery w/ tools like telnet), or you just get lucky w/ a Google search, you're pretty much left to cracking the case and taking a peek at the PCB.
FWIW, devices provided by the ISP are typically quite lame. Let’s face it, for them, the cheaper the better. And cheap translates to weak processors (200-266MHz), modest ram (4-16MB), modest flash (2-4MB). If you really need performance, there’s no subsitute for one designed for that purpose. Just compare those crappy specs to the ASUS RT-N16 router; 533MHz processor, 128MB ram, 32MB flash.