Majority of the time, you will notice some discoloration (black/brown) on the tooth, which will be associated with food getting stuck over that area.
Sensitivity / mild pain if the cavity is superficial and if the cavity is deep you might get moderate to severe pain.
Next Steps
So if you notice any of the above mentioned symptoms you better visit a dentist
Health Tips
If the filling is superficial dentist will suggest you a filling, if it is deep you have to go for root canal treatment