If you are having application performance issues and how cab we figure out it is related to disk / SAN issues? If write cache is enabled on operating system / disk for DAS or on SAN, writes from a database should be very fast and with a consistent response.
The best metric in perf mon is to measure the avg disk sec / write i.e the amount of time it took for a write to be completed by the disk or SAN. Generally a write should be with in 1ms - 5ms [microsoft recommendation] especially for transaction log.
If the write response is jumping all over the place and is not consistent over time, its time to look into SAN or disk performance.