How to identify the address where the test is done?

[t3xc0d3]

You can set a read/write hardware breakpoint to obtain the location that reads/writes the string.

Another possibility is a pure static approach: searching for xrefs in the code. Doing that, you will see that 0x7B31B6 loads the data location into eax and then calls 0x40A748.

[byvs]

And how do I stop this test and change the string?

[bongos_man]

assuming the program is otherwise unprotected and will not try to prevent or detect it, write a loader which injects a dll into the target process's memory and patches bytes in the appropriate place to call a function in your dll that changes the string however you wish. there are lots of tutorials on code injection, here are some good ones:

Three Ways to Inject Your Code into Another Process
A More Complete DLL Injection Solution Using CreateRemoteThread
Code Injection - A Generic Approach for 32bit and 64bit Versions
InjLib - A library that implements remote code injection for all Windows versions

[byvs]

Originally Posted by bongos_man

assuming the program is otherwise unprotected and will not try to prevent or detect it, write a loader which injects a dll into the target process's memory and patches bytes in the appropriate place to call a function in your dll that changes the string however you wish. there are lots of tutorials on code injection, here are some good ones:

Three Ways to Inject Your Code into Another Process
A More Complete DLL Injection Solution Using CreateRemoteThread
Code Injection - A Generic Approach for 32bit and 64bit Versions
InjLib - A library that implements remote code injection for all Windows versions

But in practice how do I stop this test and change the string?