If I recall correctly the 289 HiPo has thicker main bearing webs in the block and thicker main bearing caps.
On the heads look for the number 19, 20, or 21 cast at the very left hand end. If not, it's not a HiPo head.
Also, if the crank is original (untouched) it should have bright orange paint on the middle counterweight. That denotes a nodular iron crank and has been Brinell tested.