Look at the clef of the piece. The clef determines the notes that exist on the staff; treble clef, a sign resembling an ampersand, has E as the lowest line note and F as the highest. Bass clef, a sign resembling a curve with two dots to the right, has G as its lowest line note and A as its highest.
Count the number of ledger lines above or below the staff.
For each line, count two notes. For example, counting below the treble staff, the first ledger line is C and the second is A. Above the staff, the first line is A and the second is C.
Count a note for spaces past ledger notes.