How to detect that the DrawerLayout started opening?

Currently accepted answer by Pavel Dudka is already deprecated. Please use mDrawerLayout.addDrawerListener() method instead to set a listener.

mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mDrawerLayout.addDrawerListener(new DrawerLayout.DrawerListener() {

        @Override
        public void onDrawerSlide(View drawerView, float slideOffset) {
            //Called when a drawer's position changes.
        }

        @Override
        public void onDrawerOpened(View drawerView) {
            //Called when a drawer has settled in a completely open state.
            //The drawer is interactive at this point.
            // If you have 2 drawers (left and right) you can distinguish 
            // them by using id of the drawerView. int id = drawerView.getId(); 
            // id will be your layout's id: for example R.id.left_drawer            
        }

        @Override
        public void onDrawerClosed(View drawerView) {
            // Called when a drawer has settled in a completely closed state.
        }

        @Override
        public void onDrawerStateChanged(int newState) {
            // Called when the drawer motion state changes. The new state will be one of STATE_IDLE, STATE_DRAGGING or STATE_SETTLING.
        }
    });

Works perfectly. Cheers!

DEPRECATED: See other answers for a more suitable solution

There are 2 possible ways to do that:

1. Use onDrawerSlide(View drawerView, float slideOffset) callback

slideOffset changes from 0 to 1. 1 means it is completely open, 0 - closed.

Once offset changes from 0 to !0 - it means it started opening process. Something like:

mDrawerToggle = new ActionBarDrawerToggle(
        this,                 
        mDrawerLayout,        
        R.drawable.ic_drawer,  
        R.string.drawer_open,  
        R.string.drawer_close  
) {

    @Override
    public void onDrawerSlide(View drawerView, float slideOffset) {
        if (slideOffset == 0
                && getActionBar().getNavigationMode() == ActionBar.NAVIGATION_MODE_STANDARD) {
            // drawer closed
            getActionBar()
                    .setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
            invalidateOptionsMenu();
        } else if (slideOffset != 0
                && getActionBar().getNavigationMode() == ActionBar.NAVIGATION_MODE_TABS) {
            // started opening
            getActionBar()
                    .setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
            invalidateOptionsMenu();
        }
        super.onDrawerSlide(drawerView, slideOffset);
    }
};
mDrawerLayout.setDrawerListener(mDrawerToggle);

2. Use onDrawerStateChanged(int newState) callback

You need to listen to STATE_SETTLING states - this state is reported whenever drawer starts moving (either opens or closes). So once you see this state - check whether drawer is opened now and act accordingly:

mDrawerToggle = new ActionBarDrawerToggle(
        this,                 
        mDrawerLayout,        
        R.drawable.ic_drawer,  
        R.string.drawer_open,  
        R.string.drawer_close  
) {
    @Override
    public void onDrawerStateChanged(int newState) {
        if (newState == DrawerLayout.STATE_SETTLING) {
            if (!isDrawerOpen()) {
                // starts opening
                getActionBar()
                        .setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
            } else {
                // closing drawer
                getActionBar()
                        .setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
            }
            invalidateOptionsMenu();
        }
    }
};
mDrawerLayout.setDrawerListener(mDrawerToggle);