How do I see the winner for an STV vote?

There's no built-in STV vote counting yet. See the description on the project page:

Vote counting options

The following modules integrate third-party counting tools with Election, to provide STV and other ranked-choice counting methods:

• Election OpenSTV (requires OpenSTV)
• Election Droop (requires Droop)

Alternatively, you can use the Election Export module (bundled with Election) to export ballot files (.blt), which you can then run through OpenSTV, Droop, or other counting tools on your desktop.

I was in a bit of a hurry when I wrote that, so I'll expand here.

STV vote counting is not easy. Different organizations require specific methods of counting (e.g. Meek STV, ERS STV, Warren STV), the instructions for which can be pretty complicated to work out, and it's a big responsibility. Fortunately, the problem has been solved at least twice before, in the two Python programs I mentioned - Droop and OpenSTV.

I have tried to implement STV counting myself in PHP (see https://github.com/pjcdawkins/DrooPHP/) and I know I could make progress on that, but I can't afford to commit any time to it at the moment. Sponsorship would be welcome :)

So, meanwhile, the built-in module Election Export will let you download votes in the standard ballot file (.blt) format, which you can count at your leisure with whatever tool you choose. Or you can use my other modules Election Droop and Election OpenSTV, which simply integrate with the Droop or OpenSTV Python libraries, letting you download vote counting reports for individual positions or for the entire election at once.

I hope that helps.