If you click on the gear icon (top right of your screen) then click on About this computer. The RAM is the 2nd entry down, below the computer name.

Edit

if you run sudo lshw -class memory in your terminal, this gives you the details of all available memory.

Use sudo lshw -c memory without the -short option.

Here's an example output:

$ sudo lshw -c memory
  *-memory
       description: System Memory
       physical id: c
       slot: System board or motherboard
       size: 12GiB
     *-bank:0
          description: SODIMM DDR3 Synchronous 1600 MHz (0,6 ns)
          product: ACR16D3LS1KNG/8G
          vendor: Kingston
          physical id: 0
          serial: 16392411
          slot: ChannelA-DIMM0
          size: 8GiB
          width: 64 bits
          clock: 1600MHz (0.6ns)
     [...shortened output...]
  *-memory UNCLAIMED
       [...shortened output...]

You see the RAM type in the description field of every memory bank, like here:

description: SODIMM DDR3 Synchronous 1600 MHz (0,6 ns)