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Sorry to hear your dog is acting strange. The fact that your dog is panting and seems disoriented worries me. The fact that this behavior has lasted 3 days also has me concerned. If she is eating and drinking and not vomiting you may be able to give it another 24 hours or so before you take her in. If she is vomiting OR not eating I would taker her in as soon as possible.
To answer your questions;
How do I know if she is running a fever?
The only way to know if she has a fever is to take a rectal temperature. In the clinic we just well lubricated, digital thermometers purchased from a large box store. Most of them will give you an accurate rectal temperature in about 20-30 seconds. The normal range for dogs is 99.5-102.5.
Should I taker her to the vet?
Yes, the question is how soon. If she is have breathing difficulty, panting and disoriented, the sooner the better.
If a dog presented to my clinic with such symptoms I would do a thorough physical exam. I would also consider running some baseline tests; CBC/Chemistry, fecal sample and urine are the ones I usually start with. BUT if the physical is perfectly normal, I will let the owner decide if they want to run the tests.
I hope this helps. I hope your dog feels better soon.