Make a time machine backup.Take a look inside the last time machine backup, check if iPhoto library has the same date and the same size (in Gb) as the original iPhoto library that you can find in your home folder under images.
Or you can backup your iPhoto library (home folder/images/iPhoto library) to an eternal HDD, then delete it from your home folder, then restore the iPhoto library from time machine, and see if it contains all your photos.