Pediatrician said a fever under 3 months old is dangerous and that babies don't always show symptoms. Should I check for fever every day to be safe?

Great question. Unless your baby has risk factor for infection (prematurity, or you had Group B Strep while pregnant for example), it is not commonly recommended for routine temperature checks. If your baby feels warm, changes feeding patterns, is excessively sleepy or irritable, recital temperature screening would be wise. .
Fever. Only if the baby looks or isn't acting right. Lethargic fussy not eating vomiting or diarrhea Should u start checking temp regularly. Hope this helps.