If you have Windows Updates enabled for your Windows 7 or 8.1 installation you will get the following notification in your notification area.
Notes for testing: Test an Enterprise or Volume License to see if this notification is given (these are not eligible for free upgrades). At current Windows 10 Insiders at build 10130 are not given such a notification.
It will give you the following information on the free upgrade to Windows 10.
You can enter the details of your email address (Microsoft Account) to reserve your free upgrade.
More details are in the FAQs:
Your system will also scan for compatibility:
Ask on the forums if you have compatibility issues.