When I started playing bass there were no electronic tuners, so you had to learn by ear. Like someone said earlier it takes practice. Listen for the wave patterns. Fret the D string at the 5th fret (a G chord) then hit the open G string. If the wave pattern you hear is slow the D string is flat. If the wave pattern is fast the string is sharp. If you can’t hear the oscillating wave pattern the string is in tune. Fret the A string at the 5th fret and repeat the process.