Best way to find out read/write distribution in MySQL? People often say things like "application X is write heavy", 20/80 write vs reads, etc. 

What metric(s) is most relevant for MySQL when checking if an application is write/read heavy? 

Is checking Com_insert, etc enough, not taking into account number of rows read/modified, or is it checking iostat metrics?
An application being read/write heavy is subject to interpretation based on the following:


How many SELECTs are launched : Com_select
How many UPDATEs, DELETEs, INSERTs are launched


Com_insert
Com_delete
Com_delete_multi
Com_update
Com_update_multi

Prepared Statements (could be a bottleneck if prepared statements are not parameterized)


Com_stmt_prepare
Com_stmt_execute
Com_stmt_fetch
Com_stmt_send_long_data
Com_stmt_reset
Com_stmt_close

How much client byte traffic mysqld sees


Bytes_received
Bytes_sent

How many bytes have been read


Innodb_data_read
Innodb_data_reads

How many bytes have been written 


Innodb_data_writes
Innodb_data_written

Perhaps a combination of these


These things are meant to measure SQL command counts and the amount of data passing here and there. There are other metrics within the InnoDB storage engine that should be monitored and tuned so that any write heavy behavior is improved.

Rather than name those features of InnoDB, please go to the  Server Status Variables page and look for any status variable mentioning the Redo Log Writes and Buffer Pool read/writes.