enabling snapshot isolation level on a database - what to monitor? I am having problems with frequent deadlocks on a subscription database.

I am considering enabling the snapshot isolation level for that database.

I am aware that there is an overhead on the tempDb to maintain row versioning where writes don't block reads.

what should I monitor before and after enabling the snapshot isolation level on a specific database?
There are doubtless many blogs that discuss this, but this post by Kendra Little (at BrentOzar.com) discussed the issues that concern you.  See:

http://www.brentozar.com/archive/2013/01/implementing-snapshot-or-read-committed-snapshot-isolation-in-sql-server-a-guide/

Kendra discusses some of the problems that arise and how you can test for potential problems. 

After going through what problems SNAPSHOT isolation may cause, you need to also realize that READ COMMITTED isolation level is not as 'simple' as some might think.