Is it baby moving?

Every day I have "movement". It feels like my abdomen is having a spasm, like quickening or twitching and sometimes it's stronger than others. And then sometimes I feel a little thump. I put my hand there and It feels like a small baby kick. And when I just look at my stomach I can actually see it. I've been having it since about 15 weeks but it's gotten more frequent over time, and it's on and off all day. I'm now 18 weeks. Is it baby moving or gas? How can you tell the difference?