All Alfa’s after 2005 had the chassis number etched into the original windscreen.
On 159, Brear, Mito and Giulietta the chassis number is embossed under an access plate in the front foot well.
On the MiTo and Giulietta it is also on a chassis plate on the left hand side of the boot under the carpet.
On a 156, 147 and GT the chassis plate is on the front slam panel under the bonnet.
On the 145, 146, 155, 916 GTV, 916 Spider, 156, 147 and GT the chassis number is embossed on the right hand front strut top.
The chassis and engine number will be on your registration certificate, in the UK it is on page 2 of the V5c on the left hand side half way down (circled)
In the UK it is also on the MOT certificate (circled)