You can download the tool here: https://www.microsoft.com/en-us/download/details.aspx?id=53598
The SharePoint Migration assessment tool (SMAT) is a simple command line executable that will scan the contents of your SharePoint farm to help identify the impact of migrating your server to SharePoint Online with Office 365. Because the tool is designed to run with out impacting your environment, you may observe the tool requires one to two days to complete a scan of your environment. During this time, the tool will report process in the console window. After the scan is complete, you can find output files in the tools directory that will give you summary and more detailed insights into the scenarios that could be impacted by migration. To improve the quality of Microsoft products and services, the tool will report statistical information back to Microsoft. Optionally, you can identify your organization as well. If the tool is not able to connect to the internet to report this information, the tool will still function as otherwise expected.