They can't tell you with words, but you can spot the signs of illness and injury in your pets.

Those big brown eyes can melt your heart, but they can’t tell you what’s wrong. Even at the best of times, life with pets can be a bit of a guessing game — but it can be scary when something is wrong. Pets can’t tell us if they’re sick or in pain, so it’s up to us to watch out for warning signs.

Some signs are obvious — like a wound or lump — but others can be subtle or confusing. Not sure when to call the vet? Here are some signs that could signal serious illness, according to WebMD and other pet health sources.

Itching and biting. Many of the things that make our pets itch make us squirm too — like fleas, ticks and mites. However, some causes aren’t so obvious, like hormonal problems, staph infections, yeast infections or fungal infections. Watch out for signs of scratching, licking and chewing — especially if there’s hair loss, scratches or skin irritation around the site.

Vomiting or diarrhea. It’s not unusual for a dog or cat to eat something they shouldn’t have, throw up and be back to their normal selves in no time. When should owners worry? When vomiting or diarrhea persists — like throwing up several times in a day — and when it’s accompanied by other symptoms like lethargy and a lack of appetite. The culprit could be a gastrointestinal illness, or it could be a parasite like roundworm.

If you see blood — whether it’s fresh or looks like coffee grounds — experts warn to call your vet right away. Blood can be a sign that your pet has eaten something dangerous (especially something sharp), but it also could be a sign of a gastric ulcer.

Lack of appetite. Whether they’re fussy eaters or bottomless pits, listen up when your pets aren’t interested in food. A lack of appetite could warn you of stress, fever or pain — and if it lasts more than a day or accompanies other symptoms, call your vet.

And cat owners take note: going without food for a couple of days can lead to an accumulation of fat in the liver. This “fatty liver” can be treated when it’s caught early, but can be fatal if left untended.

Lethargy and decreased activity. Like their humans, pets can tire out with lots of work and play — but unexplained fatigue or a drop in energy shouldn’t be dismissed. Pets may become less active as they age, but weakness, collapse or a change in how much activity your pet wants to engage in should warrant at least a phone call to the vet. Lethargy could be due to a temporary illness, or something more serious like a heart condition.