One thing that I failed to purchase in time, even though I felt that's needed, is a pulse oximeter. Which shows oxygen percentage in blood when touched on your finger.
Now in Covid, this one is the key instrument for measuring how badly one is suffering. As no other symptom is consistent other than only this one.
Some mobile phones have this feature, like the latest samsung phones, or apps or some wrist bands. But I don't have any of those. And apps from appstore might just be faking the numbers.
So here's the alternative. Poor man's oximeter. The Roth score. Take a deep breath and the count 1 to 30 loudly and fast. If you can count to 30, then you probably are fine. If not then there's a graph and score table indicating your possible oxygen level.
Link to article and graph here :
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6490799/
Note that normal oxygen level is 98%.
Safe level is 95%.
Bellow 92% and you are in danger zone, need O2 supply.