How can I see it ??

You should be able to watch it online for a small rental fee at Amazon, or buy a used copy of the DVD online for next to nothing. Manhunter isn't that obscure of a movie, you shouldn't have any trouble finding it.

As to whether you can watch it online for free, I'm not sure. Youtube once had a bootleg copy of the entire movie online, I'm not sure whether it's still there since youtube tends to delete links to full films that are under copyright.