How to determine which xcode was used to develop from code

If you open the project.pbxproj file and search for CreatedOnToolsVersion, it looks like that will give you what you want here, at least with newer versions of Xcode.

I see it set to 7.3.1 in one of my projects and 8.3.2 in another, created with that version of Xcode.

If you have .xcarchive or .ipa file that is associated with the project then you can determine the version of Xcode that was used to build it. Xcode adds, among other things, a "DTXcode" key to the info.plist upon build.

So open your .xcarchive file with Finder or rename the .ipa file to .zip and open that with Finder.
Navigate to find the app bundle file - that's the one with the .app extension. Right click and "Show Package Contents." Find the info.plist and open that with a text editor.

You will see something like:

<key>DTXcode</key>
<string>0611</string>

In this case, 611 is Xcode 6.1.1.

Alternatively, if you have access to iTunesConnect, you can see the details of a binary that was uploaded. Look for "Build Details" and find a key called "Build SDK." It will be an alphanumeric string, e.g. 12B411.

Since Apple usually only bundles one version of the SDK with Xcode, you can use this site to find the version of Xcode that corresponds to the SDK: https://en.wikipedia.org/wiki/Xcode

In the case of 12B411, I see that is was iOS 8.1 final, which was bundled with Xcode 6.1 build 6A1052d.