/Applications/Utilities
), select your startup drive in the list on the left, click the First Aid tab to the right, and then click Verify. If Disk Utility finds problems, you’ll need to boot from a different volume to perform the actual repairs using the Repair Disk button. If you’re upgrading from Lion and your Mac can use Lion Recovery (known in Mountain Lion as OS X Recovery), you can boot into recovery mode (by holding down Command+R at startup) and use Disk Utility from there; alternatively, if you created a bootable Lion-install drive for older Macs or for newer Macs, or created a separate Recovery drive, you can boot from that drive and use Disk Utility. If you’re upgrading from Snow Leopard, you can use Disk Utility when booted from either your Snow Leopard Install DVD or the OS X install DVD or thumb drive that shipped with your Mac. If you’re feeling especially cautious, you can also opt to run Apple Hardware Test./Applications/Utilities
), select Applications (under Software in the sidebar), and then click the Kind column header, which sorts the list of applications by processor type. Any programs listed as PowerPC will not work under Lion or Mountain Lion. (If you’ve got any listed as Classic, well, that ship sailed long ago.)