Find the style number
Air springs usually carry a style or part number moulded into the rubber or stamped on the bead plate. It's the most reliable way to identify a replacement.
If the number is gone
Note the suspension make and model, the mounting arrangement (studs, bolt holes, air fitting position) and the spring's design. Photos of the top plate and the piston help considerably.
Suspension work involves stored energy. Follow the vehicle manufacturer's procedures and have the work done by a qualified technician.
General information only. Always follow the vehicle and component manufacturer's documentation, and have safety-critical work carried out by a qualified technician.