A paper in the British Medical Journal, probing the details of how much physical distance is appropriate to minimize the chances of infection, introduces a simple graphic that provides a quick read on the risk across a number of dimensions.
Here is that graphic, followed by the caption provided by The BMJ (click on the graphic to open it full-size in a new window). The paper goes into much more detail on how to apply the risk scale depicted here.
Note that the graphic accounts for all the situational and environmental variables that you would be able to assess — setting (outdoors / indoors), ventilation, density, distancing, masking, droplet-producing activity (speaking, singing, shouting). It omits those variables about which you can have no knowledge — your own susceptibility to infection, and the viral load of any potential virus transmitter.
These days the journal styles itself “The BMJ.” One innovation this publication introduces that I like very much: characterizing each author in the byline. The paper is credited to Nicholas R Jones, clinical researcher, Zeshan U Qureshi, clinical academic, Robert J Temple, medical student, Jessica P J Larwood, medical student, Trisha Greenhalgh, professor, Lydia Bourouiba, professor.