Where can I get a site key / secret key?

Register at https://coinhive.com. Then get your coin hive public site key (public) and secret key from https://coinhive.com/settings/sites.

Where can I see whether miner is running?

Activate ‘log’ at Settings > WP Monero Miner using Coin Hive. Then open your page and the Chrome Console (F12 > Console). You should see the log output there:

Miner 1.2.0
Miner throttle is 0.5.
Miner autostart is enabled.
Miner cookie is set to ‘Mining allowed’.
Miner started.
Miner running with 50 Hashes/s
Miner running with 53 Hashes/s
Miner running with 47 Hashes/s

Remember to disable the ‘log’ setting again for best performance.

Note: If you have multiple tabs with your page open, the miner will only mine in one tab.