Well after a while youll learn them by sound but the easiest way is just to count, bearing in mind that except for the third (g) string, the 5th fret on each string is the same as the note of the open string below(g string its the 4th) and the 7th fret is the same as the open string above except for the second (b)string which is the 8th. Its better not to rely on positions, cos if you get into altered tunings it wont work anymore.