Use Tools > Notifications to get notified by email when someone changes the spreadsheet.
Google's Spreadsheet Notifications are very useful if you need to know when a change has been made to a Google spreadsheet you own or are collaborating on. Go to the Tools menu and select Notification rules.
There are a range of settings to choose from, giving you a while host of notification options.
It's particularly useful if you have set up a Google Form, you can be notified each time the form is completed.