You can check your holiday balance at anytime.
Head to your online On the Beach account and go to the section 'My Bookings'.
From here you'll see your holidays, if you look out for the holiday you want to check the balance for, you should see a 'Payments' button above where it says 'View and Manage booking'. Once you've clicked on the 'Payments' button, you'll be shown your outstanding balance, how much you've paid towards the holiday and your payment schedule all in one place.