Getting ratio of reads to writes on SQL Server
This will be incomplete, but it will give you a good idea of the ratios you're looking for. You can use sys.dm_db_index_usage_stats to retreive the access from various indexes on the system. If you combine the seeks, scans, and lookups you'll get a good idea of the reads. Something like this:

SELECT  ddius.user_updates,
        ddius.user_lookups + ddius.user_scans + ddius.user_seeks AS user_reads
FROM    sys.dm_db_index_usage_stats AS ddius


This data is cumulative, so you'll need to capture the previous day in order to find the differences between yesterday &amp; today. As I say, this won't give you a perfect measure, but it will move you in the right direction.