"How do you know a baby has SMA " - the only way you will be sure is by blood test. If your instincts are telling you something is NOT RIGHT, like your baby is missing milestones by large margins. Get a test.
In my case, my older child was just 16 months when Katie was born so I had a fresh comparison. Granted, all children develop differently, however Katie couldn't bend her knees by the time she was 3 weeks old. Her limbs flopped like bean bags when they were not supported and she was never able to lift her head. She could roll it back and forth, very slowly. With therapy, she did get stronger but ultimately SMA won.