If you're familiar with the iPhoto icon, you can go to Finder, which will open by default in your Home folder, and then go to Pictures. If you see 'iPhoto Library' and it picture with a palm tree and camera, iPhoto is still installed and recognized. If it isn't, the application has been deleted.
If it is indeed gone, you can, as Alistair Maxwell mentions, the Applications DVD will allow you to reinstall the iLife suite.
Failing everything else, or if you're not quite confident, you can take it to a local Apple Store where they'll reinstall iLife or recover your current application at no cost.