It should have a number or ref stamped on it, which you can look up on t'internet. The air caps, nozzles and needles work together as a combo (aka a 'setup').
There'll be specs for each gun and the setups they recommend for the various paint types.
So it comes down to what gun is it, what paint do you want to use, and what do the docs or the guys here that know the gun recommend.
E.g. for mine I wanted two or three setups. So sniffed out the info from PDFs/websites, i.e. what parts were meant for what paints. So then could suss what bits I already had, and what I needed.
They're not cheap new though.. so being a skinflint I waited till they showed up cheap.
Thank you Chipper (Devilbiss JGV) fine for using on iron/steel work but recon it would be too spattery if I ever came to use it for car pannels so thought I might invest in a new or more suitable nozzle just for that kinda work, no numbers on original nozzle. I will do a search on www though, thanks again.
No worries. Worth asking them direct, might take a bit of time but I got parts list1 and manual from them even though the guns are donkeys old, and think they answered a question or two if I remember. Pretty good service...