Even if you can't see the explosion, you can either wait for the plume of ejecta to rise up into the sunlight (soon afterwards) or reflect earthshine, which may then be visible here on earth. Or, if you have the equipment, tune your radio gear to 2235.1 MHz and watch as the signal from SMART-1 goes from on (alive) to off (dead) - several radio telescopes in Australia and Chile will be watching as the probe hits.