Slow storage writes: how many milliseconds exactly? SQL Server 2008 R2
You can query sys.dm_io_virtual_file_stats to find out the read and write latency.

You can use the script from here or here - this is snip from my project of designing a comprehensive SQL Server health checker (this is something internal for now, but planning to opensource the code soon - as a thank you to sql server community !).

Also, read this blog post by  Erin Stellato - What Virtual Filestats Do, and Do Not, Tell You About I/O Latency. This blog post has a script that you can use as well.