I can remember I just went to the toilet as I felt something in my pants and it was just like a green horrible snotty thing with bits of red in, 24 hours later and I went into labour.... exciting! But some don't lose it till there actually in labour.. some lose it days/weeks before x
Sorry if it sounds gross but its snot like. When I lost mine with my first I didn't no what it looked like so I googled mucus plug images and that's how I new what it was then.