Question: How do you know if a note is a sharp?
When the sharp sign (#) is next to the G clef and F clef, how do I know what notes in the music piece are played as sharps?
Albert's reply: One sharp is the key signature for G major and its relative key of E minor.
Here is the key signature you're talking about:
It means that every time you see any F on the staff, you need to play F-sharp instead (unless otherwise indicated).
I've highlighted the F-sharps for you in red. Notice how the sharps in the key signature are on F? This is musical shorthand indicating that all the F's are to be played as F-sharps instead, not just the ones on those two lines.
In all standard key signatures, the first sharp is always F-sharp. From there, they simply go up a fifth: The second sharp is C-sharp, the third is G-sharp, the fourth D-sharp, and so on.
Flats start with B-flat and go down a fifth: E-flat, A-flat, D-flat, and so on.
If you download the Major Scales for Piano worksheet, you can see all the sharps and flats in the key signatures in the proper order.