Once the command has started its execution, synchronously or not, the underlying os.Process gets populated and it is possible to see its PID, as we can see in the following example:

package main

import (
    "fmt"
    "os/exec"
)

func main() {
    cmd := exec.Command("ls", "-l")
    if err := cmd.Start(); err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println("Cmd: ", cmd.Args[0])
    fmt.Println("Args:", cmd.Args[1:])
    fmt.Println("PID: ", cmd.Process.Pid)
    cmd.Wait()
}