How i can check if interbase client is installed?

subj.

Are there some registry entries associated with the client? If you are not sure, you could install the client on a clean system and use RegShot to see what registry entries are created.
Or maybe there are some files that you can look for? Maybe use the locate plugin?
CF

As I guess, checking interbase client (gds32.dll ?) is very similar to checking Firebird client. To check and upgrade my Firebird client I use UpgradeDLL.nsh (or as recommended - Library.nsh)

Or, if you need just checking for existing:
1. Find in registry:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\SharedDlls]
"C:\\WINDOWS\\system32\\FBCLIENT.DLL"=dword:0000003c
2. If record was found, check existing of file in %SYSTEM% path.