When identifying which site collections are inactive you have to first define the meaning of “inactive” term. There are at least two meanings of this word:
- No one accessed site collection since X days
- No one added/changed/deleted any content in site collection, but site collection might be still used by visitors
When it comes to getting if site collection was accessed in a period of time then there is no easy way to do this. You have to dig into IIS logs, SharePoint logs, etc. Then you have to create reports from discovered logs and decide.
If you would like to know if someone made any changes in site collection then you can do this easily. SharePoint comes with three really useful properties to help you identify whether site collection was recently modified or not.
To check briefly when was the last time someone modified site collection read following properties:
On site collection level:
On site level:
You can read them more about on MSDN sites, but remember that this is only brief check. Examining these properties will give you only potential candidates for further investigation.