Generally shows in early previews (Blackbird, Disaster) have a good chance of being easier rushes, and those that have been running for a long times and don't do well at the box office (most of those closed in the beginning of January). Have a look at the latest grosses and see which shows that sell low capacity have a rush policy and a performance tonight, and be prepared to go several box offices, or have a back-up plan, i.e. a discount code that might get you a rear mezz ticket for $40-$50 dollars... or like miriam said, SRO