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What OS do you have?

This is never a good question because if you have to ask it then 90% of the time the answer is "too little!"

Step One: 32/64 Bit System?

Under windows:
http://www.howtogeek.com/howto/21726/how-do-i-know-if-im-running-32-bit-or-64-bit-windows-answers/

Under linux:
http://www.howtogeek.com/198615/how-to-check-if-your-linux-system-is-32-bit-or-64-bit/
32 you can use till 4gb.
64 you can use more then 4gb.

But now you only have to know which Motherboard/Mainboard you use.
If you can check where you bought the PC (recipe or smth) and then search for the infos about your Mainboard.

Windows
Start > Run > type "dxdiag" (no quotes) > If your system says 32-bit, 4GB. If it says 64-bit, 4GB+.

I mean it's okay because if you don't have enough RAM you can always download more!

Use CPU-Z program, it's small but let you know all about installed hardware.

If you can't see this, then most of the time only hardware that reserves RAM is integrated graphic card with shared memory. So if you have it, then it's your total memory minus memory reserved by your IGFX (which is usually between 32 and 256MB).

If you have BIOS only, you can see it when you boot up the computer: it will check the memory and list how many MB is detected.
If you have an UEFI, just go into it. Most of them list basic HW specs on their starting screen or at some Info type of screen.

I use a little program called ........ Speccy ........ It will give you all your pc info and temp

Windows key + pause break. Works on every single type of windows.

In 99.9% of cases all of your RAM installed in your rig is usable.

cpu-z or something like this is a good advice.
As since a couple of years the memory-controller is in the cpu, it´s possible that the processor can handle less banks than the mainboard.(marketing limits)
For e.g. some i3/i5 models. In those cases the full memory is recognized on startup and also recognized by OS, but it´s not fully addressable (no matter what OS). e.g. 216 GB would work 48 GB only 16 GB useable.Tools can show you this case.

Resource Monitor (Performance Monitor) perfmon.exe in system32 folder came as default with windows and its Memory tab shows a graphical representation of what part of memory is used or not by what program. To get the "usable memory" number, you should run the perfmon with no other programs running, since usable memory is equal to the sum of free + standby (will be freed in need cache) memories.