Yes, you can trust the operating system is using all that memory (if it's the 64-bit version).
The specification for this processor states that it has 2 memory channels. The datasheet for 2nd generation i5 processors also mentions that this processor family can work with 2 DIMMs per channel (supported DRAM device sizes are 1Gb, 2Gb and 4Gb, do not confuse with DIMM capacity). Check page 20 for more details.
So with 2 memory channels, 2 DIMMs per channel and the support for 8GB DIMMs (2x4Gb DRAM devices or other combination), you have a maximum memory support of 32GB.
I do not have enough knowledge about the chip industry to explain why the specification table shows a maximum of 8GB RAM but I can speculate that the internal memory controller is not the only component restricting the maximum amount of memory support. Other chip and motherboard features are likely involved. Component quality, what has been shown to work in the labs, etc, all these things will influence what a manufacturer is willing to commit to.