IsBuildRunning.cs

// Tested only on Windows
// Assumes builds are in the project folder (e.g. your-game/Builds/Build.exe)

using System;
using System.IO;
using System.Diagnostics;

public static class BuildRunning
{
  public bool IsBuildRunning()
  {
    string projectDirectory = Directory.GetParent(Application.dataPath).Name;

    foreach (Process p in Process.GetProcesses())
    {
      string fileName = Path.GetFullPath(p.MainModule.FileName);

      if (fileName.Contains(projectDirectory))
      {
        return true;
      }
    }

    return false;
  }

}