It is quite more difficult to play in tune a fretless bass than a double bass. This is because a number of different reasons: In the double bass you will use the half and first positions much more often than you would do in the electric bass, and the sound of the electric bass, just because of how it is produced and amplified, is more direct and probably louder so it is easer for the human ear to detect wether you are not in tune or not. It is just as when you play the double bass with the bow: you just can hear it much more easier than if you were playing pizzicato! In the other hand, about the scale of the fingerboard, the distances are smaller in the electric bass than in the double bass, which implies that for a same distance shift the pitch change is smaller in the double bass than in the electric bass. Obviously it makes it easier in the double bass.

A student of mine asked me once if it would be a good idea to use a chromatic tuner to learn how to play the fretless bass in tune. Well, it is always a good thing to have a chromatic tuner, specially if you don't have any other tuner, but forget to use it to learn how to play in tune because you'll get mad. Your ear should be the best tuner always. Try to play a couple of notes several times making sure that they are in tune. Then, add another one, and do the same again and again, etc. You can do this within a scale, within in a triad or within a pattern or a phrase that you like. Then build everything upon it. The clue is about your ear, and first of all you'll have to teach your ear what's right and what's wrong, that's why we need just two notes. Pau Casals used to spend hours just going from one note to another and back again until he made sure it was the right distance and the right sound. You may feel that that is something you could do watching the tuner but the interesting thing is letting your ear be the lead and see how, through the practice, in some days or in some weeks your ear starts to telling to you which is the appropriate distance and sound... Just try to play over any record that you like and sooner or later you will begin to notice some changes in your ear...and then you will hear them as well in how much in tune is the music you are playing.