The only SURE way is to measure the diameter of the journals with a micrometer. Check them for wear and "out of round".
That being said, if the engine is in good shape, and was in good running order, look closely at the outside of the old bearing inserts. They will be stamped with a part number, then "STD" for "standard" ".010 US", meaning ".010" UNDERSIZE" (or .020. .030, etc.).
Once you have the new bearings use plastigage to check oil clearance ofter installation.