How do you know that it's not Deep Vein Thrombosis but something much less serious? See detail.

If someone (around 70 yrs old) with swollen leg is panicking thinking it could be Deep Vein Thrombosis and is about to see doctor in a few days, what should he/she know that leads to no stress /panick until the visit with the doctor?
_’‘Deep vein thrombosis (DVT) and thrombophlebitis: If you have DVT, it means there’s a blood clot in a vein in your leg. It could break off and travel to your lung. When that happens, it’s called a pulmonary embolism, and it can be life-threatening.’‘__
https://www.webmd.com/dvt/why-legs-puffy