Checking database files for physical fragmentation

I searched the Internet and did not find clear explanation for when I should try defragmenting physical database files for a DB residing on a SAN. I understand that there are tools and procedures inside some SANs that deal with that, but also there are SANs which lack those features. My question has 3 parts: - How to check for physical fragmentation of database files? - Is it necessary to do this if the files reside on a SAN? - Is it necessary to create bigger physical database files to avoid fragmentation? We use a rented virtual server. The database is growing and we have some strange problems. I want to be sure that defragmentation is not an issue. Also I am just generally curious.