The final way of telling if it is genuine or not, is if the owner has old photographs of the car. Being depicted in somewhere in the 1970's, whilst on holiday etc etc. Old photographs will normally be out of focus and on 110 or similar format. Of course the fashions of the era should give you another clue! But unless the seller is an original owner, they are unlikely to have any photograph proof.
From the list below, which has taken many years to compile, you can see there are only a few genuine Brabhams remaining and a few I can't say for sure are genuine.
There is no evidence to say there was a proto-type Brabham. If any, it would have been Jack Brabham's personal car 'MO 2' or any of the advert/article cars. There is speculation that the Crayford Brabham never existed as DVLA say that registration does not exist. This could well have been a standard SL90 Crayford HB with stripes put on for the photo shoot. To date, no pictures exist showing the Brabham convertible's engine bay.

Road Test Cars
LXD 997E Dark Green?, White Stripes, Genuine Deluxe 90
KXE 905D Dark Green?, White Stripes, Genuine SL 90
LXD 992E White, Black Stripes, Genuine Deluxe 90

Jack Brabham Adverts/Articles
KXE 962D White, Black Stripe Genuine Deluxe 90
KXE 962E Touched up advert photo of KXE 962D
GXE 282E White, Black Stripe Genuine Deluxe 90 - Press Vehicle
KXD 15D White, Black stripes Genuine Deluxe 90
KXD 16D White, Black Stripes Genuine SL 90
MXD 170E Dark Green, White Stripes Genuine SL 90 Estate - Press Vehicle
MXD 183E Blue, Genuine SL 90 - Press Vehicle
MO 2 Dark Green, White Stripes Genuine SL 90 - Jack Brabhams
LNW 860F White, Black Stripes Genuine SL 90 Crayford Convertible

Known Genuine? Cars
LXD 992E White Black Stripes 100%-Genuine Deluxe 90 - Richard Holmden
XAR 931F * White Black Stripes 100%-Genuine SL 90 - Ex - Geoffrey Green
GRV 525E * White No stripes 100%-Genuine SL 90 - Ex - Geoffrey Green
OXD 422F White Black Stripes 100%-Genuine SL 90 - Edd Laxton (No longer a Brabham)
TMD 179F Dark Green No Stripes Genuine? SL 90 - Bryan Thompson.
Stripes added later on.
KCD 650F White Black Stripes 100%-Genuine SL 90 - Darryl Burrows
RXC 565E * Dark Green Black Stripes 100%-Genuine SL 90 - Seen at Motor 100,
NCH 363E Peacock Blue Black Stripes? Genuine??? Deluxe 90 - Micheal Cuthbert
HNT 626D Dark Green White Stripes Genuine SL 90 - Darrell Green.
JTA 153E * Red Black Stripes Genuine Mike Harrold.
MRJ 158F Peacock Blue White stripes 100% Genuine SL90 - Ex Deon Wilson.
HDV 740E Peacock Blue No Stripes now - Genuine SL90 - Nick Woolacott
* = Unknown if they exist anymore.

Known 'Plastic' or 'Replica' Brabhams
KJR 779D Pageant Red. Made up using Brabham parts. Uses a 1256cc engine. Originally only a basic Deluxe HB. Shell soon to become KXE 154D. Owner - Adrian Miller

OTJ 248G White SL 90. 4 Door. Made up using Brabham parts. Brabham Kit for sale by Adrian Miller in 1993/4. Article in Summer 1995 Newsletter. Owner - Dave Marriage

SBP 196E White Auto SL 90. Advertised as Auto in Dec '90 Newsletter. Made up by Adrian Miller. For sale in Autumn '93 for £1,000. David Lancefield, then converted to manual. Now owned by Peter Kemp.

NRP 177F Pageant Red. Originally a standard Deluxe HB. Made up using a set of Brabham carbs. Home made stripes. Owned & built by Mark Williams.

TJD 609F White SL90. Black stripes. Made up in late 1980s by Steve Judge. Ex Alan Anderson.

Virtually sure it was plastic
YPF 677G Dark Blue, No Stripes, but hand scripted signs above 'Viva' signs on front wings, same as Air Filters. SL 90 - 4 Door. Scrapped in a Banger Race in Oxfordshire.