How to know a buffered channel is full

You can use the select statement with a default. In case it is not possible to do any of the cases, like sending to a full channel, the statement will do the default:

package main

import "fmt"

func main() {
    ch := make(chan int, 1)

    // Fill it up
    ch <- 1

    select {
    case ch <- 2: // Put 2 in the channel unless it is full
    default:
        fmt.Println("Channel full. Discarding value")
    }
}

Output:

Channel full. Discarding value

Playground: http://play.golang.org/p/1QOLbj2Kz2

Check without sending

It is also possible to check the number of elements queued in a channel by using len(ch), as stated in the Go specifications.
This in combination with cap allows us to check if a channel is full without sending any data.

if len(ch) == cap(ch) {
    // Channel was full, but might not be by now
} else {
    // Channel wasn't full, but might be by now
}

Note that the result of the comparison may be invalid by the time you enter the if block