How would I know if I had caught smallpox?

Dreadfull illness. Adults with smallpox were dreadfully ill within days, bedridden with high fever and loss of energy. Since no wild cases have occurred in the US since 1949 and worldwide since 1977 you are not likely to catch this in your lifetime. See the CDC website for specific details.
Eradicated. The WHO announced worldwide eradication of smallpox on May 8, 1980. Smallpox has no animal reservoirs. So smallpox would not return naturally. So, you wouldn't catch it. However, according to the CDC, “The U.S. government has enough vaccine to vaccinate every person in the United States in the event of a smallpox emergency.” This is part of Emergency Prepared & Response.
You didn't. Trust me...You don't have it. If you're going to obsess about a deadly disease, you should obsess about one that actually exists on the planet..