Toxoplasmosis is actually caused by a parasite named Toxoplasma gondii. Dogs become infected (usually) through eating uncooked meat or being exposed to cat feces.
Symptoms are pretty general - diarrhea, fever, etc - but a wide variety of severe symptoms can emerge. It would be a very lucky guess to know a dog had T. gondii just from a symptom, and so it's most normally diagnosed through testing at a vet, by looking for parasites in the blood.
In humans, toxoplasmosis is particularly harmful to people who are immunocompromised (cancer patients, AIDs patients, etc).
I found a website with a much more in-depth explanation if you're interested in knowing more: