I've been wondering this exact same thing. I had bubbles at the start of 15 weeks so I figured that was the baby. A few days later I started getting a feeling like an internal poke, it was really gentle but it was like a short repeated poke, poke, poke. It went on for about half a minute. I thought it was probably wishful thinking as I didn't think I'd 'popped' yet and the feeling was an inch below my belly button but my midwife's appointment the next day confirmed that was where the baby is. By the end of the week I was feeling the poke more often and a bit stronger. I was at a comedy gig and I more or less missed the grand finale as I was feeling this persistent poke, poke, poke for about 5 minutes and it completely distracted me.
Then I felt nothing for 6 days so I thought I must have been wrong about it being the baby, though it had definitely found my bladder as I started experiencing the sudden desperate need to pee even and then not needing to go once I'd gotten to the toilet. But it started up again today, it was strong enough to startle me but only because I was sitting down at the time. Tonight I felt a similar feeling in a different spot but had gas straight after, which happened a few different times, so I'm not sure any more. I just wish I knew. It's very deflating to feel a poke, swell up with love for your tiny, kicking baby and then break wind.