How can I tell if what my baby has is just a cold?

Just a cold? The best way to tell if it's just a cold is by how long the symptoms last. There should be stuffy, runny nose for anywhere from 3 - 7 days. Eyes may water, a sore throat may follow, but symptoms should clear by seven days. A fever may be part of the illness. If fussiness increases, go see your pediatrician or family doctor to make sure an ear infection is not complicating the cold.
Congestion is key. The main way to tell if it is just a cold is by congestion. This presents itself by a runny nose. The baby usually does not have a fever. Is still eating well and otherwise is behaving the same. If these other symptoms are occurring. Then the mild congestion may have progressed into sometime more serious like a bacterial ear or throat infection which may require antibiotics. Dr. Gerard roberts.
Mild congestion. If your baby has mild congestion (the mucus can be clear, slightly cloudy or even yellow) and possibly even a slight cough due to the postnasal drainage, your baby likely has just a cold. If your baby has labored breathing, frequent vomiting from cough, irritability, poor feeding/poor sleep, or very high fevers, there may be something else going on. If that is the case, check with your doctor.
If acts ill, see doc. Telling a cold from other causes of upper respiratory troubles can be tricky. Allergies run in the family? Smoke exposure in the home? Other irritants that can cause respiratory problems? All of these may contribute to baby cold-like symptoms. Really, until your child is older and you have a good feel for their tendencies and traits, it is probably wisest to have your pediatrician evaluate them.