platform/core-api/src/com/intellij/openapi/project/ProjectCoreUtil.java

package com.intellij.openapi.project;

import com.intellij.openapi.fileTypes.FileType;
import com.intellij.openapi.fileTypes.InternalFileType;
import com.intellij.openapi.vfs.VirtualFile;

/**
 * Author: dmitrylomov
 */
public class ProjectCoreUtil {
  public static final String DIRECTORY_BASED_PROJECT_DIR = ".idea";

  public static boolean isProjectOrWorkspaceFile(final VirtualFile file) {
    return isProjectOrWorkspaceFile(file, file.getFileType());
  }

  public static boolean isProjectOrWorkspaceFile(final VirtualFile file,
                                                 final FileType fileType) {
    if (fileType instanceof InternalFileType) return true;
    VirtualFile parent = file.getParent();
    while(parent != null) {
      if (DIRECTORY_BASED_PROJECT_DIR.equals(parent.getName())) return true;
      parent = parent.getParent();
    }
    return false;
  }
}