Hi T,
Ask some of your old coworkers about where they all tend to hang out after work.
If they let you know that they will be meeting up at a particular location after work on a specific day and R is also going to be joining them, you can go to the location before they arrive and find a spot.
It may be best to go there with one of your friends so that you don’t feel so nervous and for some support. When your old coworkers show up, you could easily start talking to them.
R may notice you soon after and may start talking to you in an attempt to catch up. This is where the both of you can engage in extended conversation and see whether there is any chemistry.