Easy – you look for one with lots of rubbish on the floor.
It's common practice for Spanish diners in tapas bars to casually drop paper napkins, olive stones, empty sugar sachets, snail shells and those gnarly bits at the end of pork sausages on the floor. Rather than an uncouth display of bad manners, it's accepted and even encouraged, an indication of gastronomical greatness in a tapas bar.
The reason for the tradition is lost in the mist of time, although some maintain that tapas bar owners have long encouraged the practice since it shows to the world how busy and popular their bar is.
In a busy tapas bar, the reasoning goes, the staff are kept busy making and serving their delicious bite-sized snacks and they don't have time to sweep the floor.
Therefore, if you want to sample authentic tapas, the messier the bar the better.
Note that untidy behaviour applies strictly to bars. Flick your prawn shells on the floor in a Spanish restaurant and your waiter won't hold back.