What we do have to judge on is the Labour Party's manifesto under Corbyn, which was a Social Democratic manifesto, and not really at all radical, when it came down to it!
Rather than who's grave he's visited, that's a far more appropriate document to see what kind of Prime Minister he'd be, surely... and that manifesto would still not be for many.
I'd hope, however, the commitment to policy would be of more importance than where they've visited although I dunno, is everybody who's visited the British Museum Reading Room a closet Marxist really?