Check if SQL Server Client Is Installed

It can be gotten through req query

reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft /f Native*Client /k

result

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server Native Client 11.0
End of search: 1 match(es) found.

then you can get exact number by parsing, for example, in python

import os
import re
os.system("reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft /f Native*Client /k > C:\\reg.out")
out=open("C:\\reg.out").read()
ver=re.search("Native Client (\d+)").group(1)
print(ver)

And you get ver=11 There is also faster way

code = os.system("reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft /f Native*Client*11 /k > null")
code == 0 # means there is client of version 11
code == 1 # means there is no such client

This can be found by running odbcad32.exe.

From the Microsoft Support site:

To determine the version of SQL Server Native Client, use one of the following methods:

Method 1: On the system where you want to find the version of Native Client, start the ODBC Administrator (odbcad32.exe), and then check the Version column under the Drivers tab.

On 64-bit systems, also check odbcad64.exe.

Go to->Run->cmd->type regedit.exe and you see the registry under the ODBC folder:

HKEY_LOCAL_MACHINE\SOFTWARE\ODBC\ODBCINST.INI\
    ODBC Drivers\SQL Server

I also find a complete answer about this issue, (based on MySQL, but basically is the same thing).

Edit later: according to your comment you have installed the 2005 version of SQL Server:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Microsoft SQL Native Client\CurrentVersion

Anyway I also show you how you can see the 2008 version to see the differences between the two versions.

For SQL Server 2008/R2:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Microsoft SQL Server\SQLNCLI10\CurrentVersion

For SQL Server 2012:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Microsoft SQL Server\SQLNCLI11\CurrentVersion