What are the signs that a baby is not experiencing normal development?

Marching Forward. In addition to dr gillilands answer, your baby should be making steady progress in all milestone categories whether it is speech, gross motor, etc. If there is no progress, there may be a problem. Regular visits with the pediatrician should pick this up.
Noninteractive. Early signs of abnormal development include lack of social smile by 2 months, inability to track objects, lack of reaction to sound, poor muscle tone (laying like a rag doll instead of curling in your arms), inability to roll over by 5-6 months, not reaching out to grab objects by 4-5 months.
Developmental . Surveillance with standardized screening tests at all well-baby checks & autism-specific screens at 18 & 24 mos. Is recommended by the american academy of pediatrics. If not done, you can use the screens at firstsigns.Org & aap.Org. As stated, progress in cognition, receptive/expressive language, fine/gross motor, self-help/ adaptive & social skills tells you if there's a problem in any domain.