Wooo this is a complicated one. You can use both but you can always say it like “how did you know i was here?” No matter the tense, in written or vocal English that will always translate well. Even if you were at the place 3 weeks ago... or if you ran into someone while at the place. How do you know i am sick? (Works for being SICK RIGHT NOW) How did you know i was sick? (Sick right now, or sick 3 weeks ago works for both :-)