For myself... I usually know what's going on.
For example... I get pneumonia every winter. Some years are worse than others. I have a lung capacity of about 3x that of a normal person (even smoking, I can still hold my breath for 2 minutes). So I just chill out at home and take care of myself UNLESS I get a fever over 103 or my lungs fill up halfway with fluid. Last time that happened "my" doc was on holiday, and the sub didn't know me well enough. Listened and "clear" (aka too plugged to wheeze), and the air meter read "normal / good". I had to insist on an xray and he did it only after argument. Sure enough... my lungs were almost 2/3s occluded. He freaked a little bit.
I'm a strep carrier (apx 80% of those who work in healthcare are either staph or strep carriers... our systems just get used to being hosts). I know immediately when the strep has decided to try a new advance. So if salt water doesn't take care of it (strep is halophobic), I pop round for a z-pac.
If I KNOW what it is, I just pop into my doc's office in the morning. He's gotten used to me. If I go in saying x, he'll write me a scrip -or call up the specialist to get me there in the afternoon-, and then run labs "just to make sure". It's always been what I've thought it probably was.
If I DON'T know what it is... I go to the ER. Because they mystery things always happen over the weekend, don't they?