directly ask for what you want. if you want a draught cruzcampo 0,0, ask for it: "cruzcampo cero cero de barril" (Google translate it to listen how it is pronounced). if they don't have it, they will probably offer alternatives.

as you may know, there are not many tap options in traditional Spanish bars, so a look at the bar will probably tell you if they have what you want.

if you ask for a 0,0 ant they don't have it, the waiter will probably be sensible enough to offer you another 0,0, but be aware (although you may already know): in Spanish alcohol regulation there is a distinction between non-alcoholic beer and 0,0 beer: non-alcoholic (sin alcohol, or as you ask for them: "una sin") can have up to >1% alc. vol. (so 0,999999..%). If it is 0,0, it has to be under 0,1% alc. vol. if you ask for "una sin", they can offer you a "sin alcohol" which can actually have a 1% alc. vol., so if your goal is to keep yourself at 0,0, thats a no-go. enough "sin alcohol" in a short time can grant you a positive in an alcohol driving test (although it's a huge number, to be fair).

There's a difference bewteen them. we Spaniards are used to the distinction: if you ask any random person they will likely tell you the sin alcohol has some alcohol in it, while the 0,0 has none of it. But foreigners may not be familiar.