How to detect hardware keyboard presence?

To detect common qwerty keyboard connected use this:

private boolean isKeyboardConnected() {
    return getResources().getConfiguration().keyboard == KEYBOARD_QWERTY;
}

[android-developers] Re: Detect Physical Keyboard Layout (ex: QWERTY vs QWERTZ)

The flags provided by getResources().getConfiguration().keyboard are a good way of checking which keyboard (if any) is available.

Use the following method to ascertain presence of hard keyboard at any time:
(To my knowledge, soft keyboards all lack the features tested below )

public static boolean isHardKB(Context ctx) {
    Configuration cf = ctx.getResources().getConfiguration();
    return cf.navigation==Configuration.NAVIGATION_DPAD
        || cf.navigation==Configuration.NAVIGATION_TRACKBALL
        || cf.navigation==Configuration.NAVIGATION_WHEEL;
}

Optionally trap all run-time keyboard changes for each affected Activity via AndroidManifest:

android:configChanges="keyboard|keyboardHidden|navigation"

But be sure to support the above manifest change with (at least) a dummy onConfigurationChanged()

@Override
public void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);
    // Optionally employ 'isHardKB()'   
}