Google them. Go scan their list of publications to get an idea of their area. Maybe glance at the more recent articles. Look them up on the Mathematical Genealogy website to see what their PhD was in.
Look for connections. Have they done something that is connected to what you do? Is their research style similar or different? Are they doing something you have always wanted to learn or you're confused about? Ask them about it!
Tell them about your research. Give them a nice gentle introduction to what you do. Tell them about interesting maths or physical problems you're working on. Tailor this dependent on what they do.
Ask them what they're working on.
Ask them about their PhD or their career. E.g. “I saw that you did your PhD on X, Y, Z with A”. How did you get to Institution B after that?“ “How did you like your PhD at Institution C? What was it like working with A?”
Ask them about their university or research group. E.g. “What is the fluid dyanamics group at Institution B like?” Ask them about what the PhD students are like, whether the undergraduates are annoying, what classes they're dealing with this term, etc.
Ask about cultural differences. E.g. “What was it like doing a PhD in the UK and now working in the US?”