How can I tell if my context is still valid?

If your Context is an Activity, you can check if it is finishing or has finished with the isFinishing() method:

if ( context instanceof Activity ) {
    Activity activity = (Activity)context;
    if ( activity.isFinishing() ) {
        return;
    }
}
Toast.makeText(context, "I'll do things here that depend on my context and views being valid", Toast.LENGTH_SHORT).show();

What you really want to use is an AsyncTaskLoader. These are my new favorite classes in the Android API. I use them all the time and they were made to solve problems just like this. You won't have to worry about when to stop your download or anything like that. All the threading logic is taken care of for you, including telling the thread to stop if the activity has been closed. Just say what it is you want to do in the loadInBackground() method. Note that if you are developing for an API lower than 3.0, you can still access all the loaders via the Android Support Package.