Motor delays are the
easiest to spot. A full-term infant can turn his head side-to-side, a 2-month-old gets his head up like a turtle & a 4- month-old gets head & chest up in prone.
"Fixing" on mom's face from ~12" away by 1 month, turning to mom's voice & following mom's face 180 degrees while she's silent at 2 months.& smiling, gooing & cooing reciprocally at 2-4 mo are important. See firstsigns.org for screenings.