Hello, im having the same problem as all audi a3’s with BKD engine from 2006 and below, i have a B letter cylinder head and i know its cracked for sure, because antifreeze is getting in the first cylinder, i know that A and B cylinder heads are way weaker than the C ones. 
 

So my question is how can i tell its a C cylinder head, where is it located on the head exactly? Would appreciate if someone could show me it with a photo, so that it would be easier for me to locate it when buying the head.

Any help would be very appreciated.

Can only presume you mean the part number casting that’s usually above the exhaust port, after the numbers you get a letter.  B supersedes A and C supersedes A and B and so on upto the designation that Audi are currently distributing