Hi Ian,
It helps to have subscribe to a decent database which gives the hex/registration tie up. Even without a database you can use online sources such as Airframes.org, GAS etc.
The registers of some countries sometimes follow a sequence and you can work out registrations and also some countries such as the UK give the hex/registration on the online registers. Other times its via forums such as mode_s, using websites such as Skyliners to assist in identifying new deliveries and putting 2 and 2 together and hopefully getting 4
Alan