I don't think the engine in my 1972 SS/RS is the original (it has K code in the VIN meaning the 350 motor) but the code on the block is T0523CTX.

This means Tonawanda plant May 23rd but my research on here shows that the CTX code is either a from a 1970 350 with manual transmission
see here http://www.nastyz28.com/gm-chevy-codes/chevy-engine-codes-suffix-13.php

the code also appears here http://www.nastyz28.com/gm-chevy-codes/chevy-engine-codes-suffix-3.php which means either a 1975 400 or 1978 305

How can I tell what it is?

Look on the back of the block. You should have a casting date E230 maybe. Which breaks down as E being May (the 5th letter in the alphabet) and then the cast day of that month. The last digit will be the year. 0 being 70, 2 being 72.