Welcome to BYC. I have 21 ducks and most of my males all ended up with the olive green bills. That was always my first indicator but that does not work with all breeds. Hopefully someone on here can give you more information. They can keep us guessing for a while for sure sometimes. lol
I concur, photos in daylight and perhaps a video. Alternatively, pick them up, one at a time, and walk away. The captive duck should make it's gender known!