All bass guitars should be tuned so that the open strings center on defined tones (usually [B]EADG[C], but drop-D and other tunings are common). This is true regardless of whether the instrument is fretted or fretless.
Then, making sure you're in tune on a fretless instrument (whether that's a fretless bass guitar or a double bass) is about hearing the note you're playing more than looking at the fretboard.
If you have fretlines on your fretless bass, you will normally find yourself putting your fingers either over or just behind the fretlines to get the note in tune. This is very similar to the proper technique on a fretted bass; you place the finger just behind the fret, which allows for buzz-free notes without bending the string sharp due to overpressure. As such, it's a good place to start when learning fretless bass.
However, many fretless basses have no fretlines; then it's purely by ear and by feel. The only hint you will get about relative position is in the harmonic points placed on the side of the neck (these are usually still present even on unlined fretless basses). The harmonic point should be right on that marker, unlike in fretted and lined fretless instruments where the dot is in the middle of the space just behind the fret or fretline that is the actual harmonic point.