Another sure way to tell: Z4 M Roadsters were only made from 2006 through 2008.
Oh yeah, that and the ///M logo/badges on each front fender and on the trunk lid, and on the steering wheel, gauge cluster, gear shift knob, and the headrests, on the door sills, on the wheels, and most importantly, on the engine. Kind of hard to miss.
And you don't just have a "convertable", you have a roadster.
Mine: 2006 Z4 Roadster (E85) | Interlagos Blue Metallic | Black Extended Nappa Leather | Carbon Leather Trim | Purchased 7/19/12
Hers: 2011 335is Coupe (E92) | Le Mans Blue Metallic | Black Dakota Leather | Dark Glacier Aluminum | 6MT | ED 5/12/11
Also Mine: (On Order) 2016 M4 (F82) | Sakhir Orange Metallic | Black Full Merino Leather | CF Trim | M DCT | Adaptive M Suspension | 19" Black Alloy Wheels | ZEC | ZLP | ZDB | ED 5/13/16