This is such a basic functionality, it really is incomprehensible that it's still not available.

If you want to search a segment to see if a particular contact is in it, currently the ONLY way to do this is to export the segment to a spreadsheet and then manually search for the contact in Excel. Why? There is a name and email column, but both are greyed out. You can filter many other parts of the marketing app and it's essential for marketers to be able to filter a segment by name and email.