How to check size of websocket compressed message in the browser

I have a simple websocket connection using the Golang library - gorilla/websocket. I have enabled message compression:

var upgrader = &websocket.Upgrader{
    EnableCompression: true,
}
conn, err := upgrader.Upgrade(w, r, nil)
_ = conn.SetCompressionLevel(9) // max level of compression
...

And I want to check if my messages are compressed, as far as I know gorilla/websocket library is making the compression and there is no information from inside and we got the real size message at the end.

I tried to check some headers and sizes from the network menu in Chrome Dev Tool, but cannot find anything related to compression size.