When you know what a "normal" throat looks like, it's usually pretty easy to tell when something is majorly off.
My son is a sleep-mouth-breather with seasonal allergies, so he often complains of a sore throat in the morning and I tell him to drink something and give it a half an hour -- usually once he starts breathing through his nose and has rehydrated the tissues, the soreness resolves. One Saturday morning he told me his throat hurt and I gave him my usual advice. He spent the rest of the day eating/drinking normally, playing with friends, playing video games, etc. -- no more mention of pain.
That evening, twelve hours later, he asked if he could spend the night with a neighborhood friend. I asked him if his throat still hurt, and he said, "yeah".
So I grabbed a flashlight and had him open up for a look and OMG!
There was every color of the rainbow living on his tonsils, with clumps and goop and all kinds of nasty going on in there.
And since nobody in my family ever gets sick M-F/9-5, we had to go off to the ER for abx on a Saturday night. I *hate* using the ER for PCP things, but our town doesn't have an Urgent Care Clinic, and there was no way in heck I was going to wait 36+ hours with that kind of toxic ooze living in my son's head!
Anyway, long story short, when there's something major going on, chances are pretty good that you're going to see that things aren't status quo. And some things just aren't obvious on kids -- that same son, when he was 3yo, ended up with pneumonia that the pediatrician initially diagnosed as bronchiolitis. It happens.