If you want to keep it within IRC, then the only way to monitor for Bits is to send the CAP REQ: "twitch.tv/tags". So when someone sends bits, there is a bit tag shown, below is a example response with tags (there is more that is sent, but I have removed extra stuff we are not looking for.):
@badges=subscriber/6,bits/1000;bits=100;color=;display-name=Larklen;emotes=; :larklen!larklen@larklen.tmi.twitch.tv PRIVMSG :cheer100
The ‘bits/1000’ shows what bits badge the user currently has and the tag ‘bits=100’ is how many bits where sent in that message (we can see the user sent the message ‘cheer100’.
So on your end, you would monitor for valid bits on a per message basis.
The alternative to IRC tags is the Twitch PubSun system found here but it does not go through IRC.