You Can't.....You can find out what it was from the facvtory by the suffix on the front pad, but you have no way of knowing(without pulling a head or pan) what crank someone has put in it since,other than the balancer and flexplate.... If that is a 512 block I WOULD NOT have it in a hotrod.
Depending on the casting date they can bring big bucks. If it has a late 1969 cast date then it is a ls5 or ls6...chevelle and vette guys will pay big bucks for it if the dates are right ...I have 2 of them.One IS A Dec 68 cast 427 4 bolt main and one is a December 69 LS5 2 bolt main...It came in a chevelle with a manual trans.....It is damaged from a lifter failure because I didn't know it was worth anything and had it in my race car...
http://www.chevelles.com/forums/showthread.php?t=289174
I think they used the 512 block for several years after 70 so it may be just another 454 but I would look at the dates on the block and see.