Any situation where you feel that silence would be less enjoyable than having a conversation, and the other person isn't doing something else that is taking their attention (on the phone, texting, reading a book, etc).

Just make a comment about literally anything and be aware of how they react to see if they want to talk back with you. Do they smile when they answer and ask a question back? Then at the very least they most likely won't mind having a convo. Do they give you a 1-2 word answer and then look away/look at their phone. Then they probably would like to be left alone.

Having conversations with strangers can be awesome and you can meet lifelong friends from it. You can also piss people off entirely if they're having a bad day and just want to be left alone but you won't shut up- just pay attention to how they react.

Anecdotally- I met a girl I ended up dating for a few months while walking from the train to a friends house. We were walking the same direction for over 4 blocks just a few yards from each other not saying anything. I felt super awkward the whole time and kept thinking I should just start power walking to speed up and get away from the awkwardness of the situation. Finally, while waiting for a walk symbol at an intersection and after 15 seconds of awkward silence, I made probably the dumbest joke ever about her helping me 'steal' a lab puppy that was being walked across the street cause it was really cute. Luckily she responded well and we ended up talking for another 4-5 blocks before we split ways after getting her number. She could have just easily given me a forced laugh and weird look before turning back towards the intersection - but my thought was that she's a stranger and I'll never see again anyways if it goes poorly so why not at least try.