how do I know if my entire laptop backed up correctly?

Used Time Machine today. Seems the memory used up in the Seagate Backup Plus devise doesn't equal the space used on my Macbook Pro. I tried to launch IPhoto from the Seagate device but it says "You can't open the application IPhoto because it is in a Time Machine backup." If that's the case, how can I tell if my backup was successful?

Thanks,
Ken