Unannounced regular random spot checks / visits , some days been in a "mood" and "have a go at them" about anything , could be the state of the van , equipment , etc , on the next visit be really nice and bring them a coffee , but "keep them on their toes"
And at the end of the day if you are not happy with them sack them , you are better of with no staff than with cr@p staff from a stress point of view........