Things to document before planning for a SharePoint Migration:

1. Document Existing SharePoint Server's Topology (preferably in Visio format, use Visio diagrams to represent the existing SharePoint Farm topology),Roles, Hardware/Software Specifications, Installed Updates, Software installed, Language Packs, etc.
2. Prepare a List of SharePoint Web Applications, Site collections, Managed paths, Host headers, Content /Service Application Databases, service application proxies & service application groups.
3. Document Central Administration Settings like User Policies, Quota Templates, Data connections, Zones, AAM, Quota Templates, Blocked File Types, Log file paths, etc.
4. Document All Third-party Software installed on top of SharePoint (Like Nintex workflows, Axceler ,Metalogix Control Point
5. Document Inventories & customizations like Solutions( wsp files), Workflows, Event Handlers, Web Parts, Features, Assemblies, Site Templates, Site Definitions, Custom Timer jobs, etc. (stsadm -o enumallwebs -includewebparts -includefeatures)
6. Document Active directory Domains/Forest details. This will help in configuring User Profile Import connection sources, People Picker configurations.
7. Document Current environment Out of the box Features enabled (Such as Publishing), Enterprise feature Enabled (such as InfoPath Farm Services)
8. Document Outgoing, Incoming E-mail settings of the SharePoint server
9. Document all IIS customizations made manually, like Web.config modifications, Add-ons installed in IIS (like Compression, URL Rewrite, HTTPHandlers etc)
10. Document SQL Server configurations, Content DB sizes, Mirroring, Clustering configuration details
11. Document any custom solutions deployed in Layout's folder or BIN folder.
12. Prepare a report on User base with Total no. of users, avg concurrent users, etc
13. Document custom scripts, automation tools you are running from SharePoint servers (like Monthly storage report generation script, scheduled with windows task scheduler)
14. Make a Note of current environment's Information Architecture, including Managed Paths, Top navigation, etc.
15. Document existing Farm's service accounts. Authentication method for all web application .Kerberos applied?
16. Document Search Settings, including content sources, schedules, Keywords, Best-bets.
17. Document Email Enabled Lists and Libraries
18. What is the current Disaster recovery plan? How often backups are taken? Third party software integrated with SharePoint (Like AvePoint, DPM, etc)
19. Document Infrastructure details such as Load balancer, DNS, IPs, SSL Certificates, Publishing configurations (Like F5, TMG, ISA Server, etc)
20. Document 12/14 hive Layout folder customizations, File system changes if any
21. Make a note of all InfoPath Form libraries. You have to update the Form Template URL and You may have to Change the data connections, etc.
22. Existing Branding Artifacts like Master pages, Themes, CSS Files, Logos, etc.
23. Document the custom authentication providers (Forms, LDAP, etc) if any
24. Document the current Monitoring setups like SCOM.
25. Last but not least - Make sure you have the version control system (like CVS, SVN, TFS, etc) which has all the artifacts including source code, installers, deployment guides.

by Vignesh Ganesan via Everyone's Blog Posts - SharePoint Community