meddyliol:
Sorry? I have no idea what you mean. I am sorry if my question is poor but I am learning the Arduino NOT an expert.
RTMS 
You get more help here if you try to figure things out for yourself.
The fiunction analogRead() is described in
Your diagram showed connections to 5V and A0 and you asked how you could tell what the input voltage is.
So what do you think analogRead(A0) does and why would the answer be multiplied by 5 and divided by 1024?
Do you know any C/C++ what does float voltage do?