AMD APU's have a kernel module that reports temperatures - k10temp. Make sure its loaded, it should show up in sensors (if not, do a 'sensors-detect' or just 'modprobe k10temp'). But temps reported by this module are not real temperatures - they are taken from the CPU temp monitor and are used internally for thermal throttling etc. On low usage they report very low temperatures, around 0 C or lower. With increased activity they become more precise, but i dont know if they will ever show real real temps.
The same goes for the integrated GPUs - they provide a sensor output through the radeon driver, but that works pretty much like the above k10temp does.
Now, if you are lucky, the mobo itself might have some supported monitoring chip - usually they are ITE chips and monitor lots of stuff, all voltage levels, fan speeds and provide a "real" temperature, usually from a probe that is located under the CPU. These chips can be seen on the mobo, they have a big ITE label then some numbers. This chip is used for the temperatures shown in the BIOS/UEFI - But keep in mind that when you are in there, there is no dynamic throttling, meaning the chip is at its top speed and will have higher temps when idling there as opposed when the OS is loaded and lowers its speed dynamically when not needed.
Unfortunately not all are supported by the it87 kernel module and from what i have heard, ITE ceased to give datasheets to the lm-sensors developers.
I previously had a Gigabyte F2A85X-D3H FM2 mobo and that had some IT8700 series it87-compatible chip. Now i have a F2A88X-D3H FM2+ mobo (with FM2 APU) and it has a chip labeled "IT8620" that isnt supported (despite its lower model number) and im stuck with the k10temp+radeon output and no voltage/fan speed readings.
As far as i remember, the k10temp module after a few minutes of high usage (using all 4 threads of my A8, for example compiling) reported 40-45 C, and the ITE chip reported something like 50-55 (with the stock fan...).
Using the open source radeon driver temperatures are lower when the CPU is in use than with fglrx mainly because the CPU doesnt use the turbo speed for some reason (with fglrx it does).