No landing clearance - don't land unless you have an emergency which prevents you from going around. Follow the relevant procedures, those which are published so that everyone involved knows what is going on.
You can't 'know' the runway is clear unless you are told by ATC that you are cleared to land. You might think you can see it's clear, and sure enough you have a great view usually allowing you to see there are no aircraft/vehicles etc on it.
But, you don't know about the large bit of metal that a ground vehicle has just spotted, or the three kids who've just jumped the perimeter fence and are heading for the runway, or the drunk on a bike who's approaching from behind a stationary vehicle, or the inexperienced pilot who's just got lost on a nearby taxiway not answering the radio, or the fact that your landing gear isn't down... need i go on? And before you ask, yes i have seen all those things happen, thankfully so far none of them combined with a radio failure aircraft which appears, despite the rules and all my expectations of what should happen, to be landing anyway!