How to tell what user has synced I have set up merge replication web sync to sync a publication from a SQL Server 2012 database.

I am trying to make some parameterised filters using SUSER_SNAME() or HOST_NAME().

When I do a sync is it possible to find somewhere what the values of SUSER_SNAME() and HOST_NAME() actually were for that sync? E.g. in the system tables.

I will need to experiment with security to work out what settings will affect these values, but I will need to find out what these values actually are to verify my settings.
Not really anything obvious.

The easiest would be if you have a backup you could restore to view it.

Otherwise you'll properly try - either yourself, daunting, or via commercial tools to find a tool that lets you explore the transaction log for the given changes.
For example something like: http://www.apexsql.com/sql_tools_log.aspx

I've not tried out such tools myself so I cannot vouch for them. 
I usually get by either by having backups to search, or in some cases, I've set up change tables to log changes to critical data.