How to identify a true Alpina?

[AlpinaMan]

How to identify a true Alpina? And what’s are the area and signs that you must look for before you purchase a genuine Alpina?

There are many times that people ask these questions but I always hear anecdotal answer such as, the front spoiler and rims, or car should have Alpina strips, or the color of the engine should be gray, or the interior should be Alpina upholstery, in which all those items could be bought over the eBay and put them on any regular BMW and make them look like one.
But what do you think in terms of technical and registered numbers that we should look for? (I will provide answer latter).