From a batch file how can I check if a program is installed?

I'm writing a batch script that needs to install Microsoft Lync if the user does not already have it installed. Then, it will install my custom Lync UI after that. What is the best 开发者_运维技巧way to check if Lync is installed considering:

1. This will run at user logon and should be fast
2. The user could be using a 32bit or 64bit Windows (XP or 7)

Check if the files either exists in program files or Program files(x86)