Well - The ASA itself cant do any check as to what machine they are connecting with and so on or even the VPN client can be anything about it.
But I can guide you in the right direction.
You can do NAC - which will allow you to use Machine Authentication.
Here is the link on Cisco connection Online that will help out with some ideas on how to implement this.
http://www.cisco.com/en/US/netsol/ns617/networking_solutions_sub_solution_home.html
Hope this helps.
Thanks
Gilbert