Same as any other instrument. You've got to play it and then decide. There are 1200 quid clunkers and 120 quid sweeties in all the shops.
My bass is a Jazz five string deluxe active. Fretless is a wolf 4 string which I Jaco'd myself - it has a beautifull Mwahhh around the 8th-10th 'fret'. (Not that it has frets of course).
John F - 27 years at the bass so far :-)