How do you know if a bite is from a spider or from a mosquito?

Duration: Likely the only way to tell will be that most mosquito bites are over/done/gone in 2-5 days. Spider bites, if from "concerning" species, will get slowly/steadily worse over that time frame, w/increasing local reaction & infection risk, and should be seen by dr if worsening after 5 days.

One vs two marks: Right after being bitten, spider bites leave two marks because spiders have 2 teeth (fangs or whatever u call it). Mosquito has only one "needle." though, it is almost impossible to see the actual puncture hole/wound from mosquito bites, and unusual for getting infected (besides malaria/encephalitis etc..). Spider bites more commonly result in local tissue injury and/or infection. Good luck.

Bites: You will be very ill if you are bitten by either the brown spider or the black widow. But they and other spider bites are very uncommon. Mosquito bites are usually slightly raised, red to pink, itchy and usually there are multiple bites.