Why do you think your opps are "supposed" to bet the river?

Judging by the betting, your opponents are drawing. Make them pay to chase; you can get more milage out of the turn.

Don't check the river. If your opponents are drawing they've missed whatever they were drawing to. Worst that happens is that they fold; you don't have to show down unless you really want to show your quad-six love pack. If they raise, you know what to do...

There is also a side point to be made: your opponents are drawing! What would you have done if a third spade came on the river? Getting money out of drawing opponents is fine but don't forget that your set on this board is by no means invincible.