If you look at the aircraft register on the CASA website, you can see who the operator is and who the registered owner is. Often the registered owner is a bank or leasing company, so that tells you it is encumbered.
A good solicitor should be able to draw up a bill of sale document that warrants it free of encumbrances. Boats are usually sold on that basis, and it's not often that buyers come unstuck. For an extra margin of safety you may need to put a broker between you and the seller and cop the extra commission. Get legal advice.