Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help now if you or someone in your care:
Get urgent medical advice the same day for severe one-sided throat pain, a muffled voice, difficulty opening the mouth, or worsening neck swelling.
These may be signs of a deeper infection.
Prompt assessment is also appropriate if you cannot drink enough, have signs of dehydration, develop a widespread rash, or become much worse after starting to improve.
Contact a healthcare professional for routine advice if the sore throat is not improving after about a week, keeps returning, or is accompanied by prolonged fever, marked tiredness, or persistently swollen glands.
Seek advice earlier if you have a weakened immune system or another condition that increases the risk of complications.
The article also says
For babies and young children, get advice if feeding or drinking drops noticeably, wet nappies become less frequent, or the child seems unusually sleepy or distressed.
Every sign above is quoted from the sofidex article on Pharyngitis - viral. If you are not sure, err upward — it is always reasonable to be checked.