Sometimes it's hard to tell if your critter is actually ill, or just acting a little sluggish, or a little silly, or just a little unusual in general. A good way to help decide is to compare his or her vital signs with the following sets of "normal" values.

Temperature: 101.0-102.0 degrees F
Respiratory Rate (RR): 10-30 breaths per minute (Panting and sniffing do not consititute abnormality in and of themselves)
Heart Rate (HR) Adult, Medium to Large Breed: 70-160 beats per minute
Puppy: 80-220 beats per minute
Toy Breeds: (Adults and Puppies): 70-180 beats per minute

Temperature: 100.5-102.5 degrees F
RR: 26-30 breaths per minute
HR: 160-240 beats per minute

Temperature: 100.8-104.0 degrees
RR: 33-36 per minute
HR: 220-250 beats per minute

Mucous membrane color (gum color) should be no paler than light pink, and capillary refill time (press on your critter's gums with a fingertip, you will see a white spot where your finger was. Measure how long it takes for that white spot to return to the color of the rest of the gums) should be no less than 1 second, and usually no more than 2 seconds. If you look at your pet's gums and you see that they are white, greyish, or blue, you have a serious emergency. Shock, severe dehydration, anemia, hypothermia, and internal bleeding are a few of the things that can cause the mucous membranes to look weird like that. Get thee to a vet!

How do you take a dog's, cat's, or ferret's temperature? Rectally. Isn't that charming? Unfortunately, it's very difficult to get the little darlings to hold the thermometers under their tongues...
Make sure you're not using an oral thermometer when you check Fluffy's temperature. (Especially one used by humans!) It's probably not a bad idea to keep a rectal thermometer (and some vaseline) on hand.