Just go to https://api.twitch.tv/kraken/streams/visceral_ and decode as JSON. If object stream is null, the stream is offline; otherwise, the stream is live.
If you’re trying to check it server-side with PHP, just Google how to decode the data as JSON and you should find what you need. It’ll come through as part of the HTML though, so it won’t be able to update live; it’ll only update once, the first time the page loads.
If you’re wanting the display to update live, without having to reload the page, then you’ll probably want to have client-side JavaScript query the API at an interval, instead of your server querying on load. Then just have the JavaScript update the display from ‘OFFLINE’ to ‘ONLINE’ or ‘LIVE NOW’ when online status changes.
To make it flash, you can either make an animated GIF, or use HTML5 to do the flashing (Google HTML5 flashing button and you should find plenty of examples).