For both of my best friend's pregnancies, I took one look at her and knew (she's not that good at keeping secrets LOL).

For my BF's sister, the first time they told his parents by using A&W Grandpa and Mama burgers like the TV commercial. They told my BF during a family dinner, very casually, like "well how do you feel about being an uncle in September?" For her 2nd, we came over to dinner and their daughter was wearing a shirt that said "I'm the BIG sister" and I was like "WHHAATT??" so that's how we knew