RAM for Optiplex 9020 SFF

Crucial's website lists that RAM as being compatible with the 9020, so that will be fine.  

I presume you've considered whether the new sticks will be added to existing ones, or will replace them?  If they're adding, you obviously need enough empty slots to take them, and I'd also make sure that they were the same speed as the original ones - the system will still run otherwise, but the memory on all banks will be limited to be as slow as the slowest stick you have fitted.

Is there a way to find out what kind of RAM stick is already installed (speed) , without the need to remove the stick to look at it?
Some software that checks all this?
,,
Yes , it will be addition to an already existing 4GB stick ..
-

yes- use siw :       http://www.gtopala.com/

or look in accessories>system tools>System info

Crucial's website also has a scanner that will tell you what you have, and also what's recommended to go with it:
http://www.crucial.com/usa/en/systemscanner

will 2 sticks of 4GB , and 2 sticks of 2GB work? or all 4 sticks need to be same size
?
?

normally - they should work together
but as said - you cannot be 100% sure, because some boards are picky about ram
best lookup what Dell says for ram configurations - see the manual also