Well, on SOME engines the location of the oil dip stick will suggest engine size. If it's on the driver's side it would be a 350, on the passenger side would be a 305. However, the ony true way to tell is to get the casting number off the block, which is located on the driver's side of the block on the very end of the motor on the bell housing flange. Once you have the number you can look it up at www.mortec.com or another site that I found http://www.nastyz28.com/sbchevy/sblock.html
Hope this helps.
Robert
74 Chevy C/10 350 c.i.d