Look at the door tops on a genuine karmann and u will see they are deeper than a standard door and the have the same profile all the way along the edge.
Other cabrio conversions that I've seen modify the existing doors and so you are left with a little 'up-sweep' above the door handle where they cut the top of the door off. Same applies to the section of body in the B-pillar area.

 

The Karmann ones are the only ones that look good. All the rest IMO look shite.

Rear working quarter windows and sill strengtheners are a good start.

 

True, True , very true

 

Genuine Karmann cab's are cut just above the decklid, and as such don't have the vented section over the engine, under where the rear screen would be. Most gashed up at home conversions retain this panel. This is the reason vented decklids were conceived originally.

And, as mentioned above, the doors are always a give-away, along with sill strengtheners, wind up rear windows, and also the top of the windscreen surround.

 

I'd say the quickest check to tell if its a Karmann Beetle is whether the rear quarter windows have chromed window frames that wind-down into the rear quarter panels, I've never seen a DIY job that has these, but I have seen a few converted saloons that at a very brief glance have looked like a genuine Cabrio!
Sure, the lack of air inlet vents under the rear window, big heater channel strengtheners and a taller door waistline are also diagnostic features of a Karmann car, but a 5-second look at the rear 1/4 windows will accurately determine whether the car is a Karmann cabrio 99.9% of the time IMHO.....
And in answer to the OP, yes there is a black enamel badge that says 'KARMANN' that is mounted low down on the offside front quarter panel (irrespective of whether car is LHD or RHD).
Hopefully you can see it in this pic;

and this is what a close-up of the badge looks like on 1961-79 models;
http://www.justaircooled.co.uk/karmann-side-emblem-beetle-cabriolet.html

 

the chassis number starts with 15.

 

But it could have been out on another chassis!

Spot-on al_kaholik, yes a Karmann cabrio post-1965 should have a chassis number that starts with the numbers '15', but there are a fair few Karmann Beetle bodies knocking around on saloon car chassis (due to age-related rust issues/accident repairs etc etc) so in isolation I wouldn't totally trust just the chassis number to determine whether the vehicle is/isn't a Karmann.......

 

But it could have been out on another chassis! The body is probably where people want a "Karmann". Nessy has it covered

 

Mine has a 150 number..look search google for chucks convertable there is a good page on wgst too look for too be a 100% sure

 

Yours is a 1970 model then (1500cc, torsion bar front suspension) .

First two numbers of a Karmann cab chassis numbers are 15, whilst the 3rd number is the model year.......so
1965=155
1966=156
1967=157
1968=158
1969=159
1970=150
1971=151 etc etc

Of course a 1975 would also have a chassis number starting 155, but that would be based on a 1303 with IRS etc , rather than the swing-axle 1300 version of 1965.........

 

Love the older verts

 

you cannot miss a genuine karmann ,plenty of reference books available that go into all the details, they have a few rust traps that you need to look out for, particulary in the UK with salt on the roads in winter, this is my 69 karmann

 

Mine a 1970 going on 1965 lol