I.G.
You can buy a cheap thermometer at any 24hr pharmacy or supermarket (baby aisle). They are less $10.
If you need to know this for an adult I honestly wouldn't bother. When I think I may be running a fever I will take two tylenol and go to bed. If your symptoms are worse than that you may consider a visit to urgent care. Most adults feel AWFUL if they are running a high fever, I mean truly AWFUL.
Good luck!