Full-back Williams, who was named in the training squad for the December 2011 Test against the Wallabies, has made 15 appearances in the RaboDirect PRO12 this season as Scarlets narrowly missed out on the play-offs.
Jones, 21, has been rewarded after an impressive season in the front row for the Welsh region and will be looking to cement a place on the three-Test tour Down Under.
But Howley admits every player will get their chance to impress in the build-up with Williams and Jones adding fresh ideas.
"To play against Australia and the brand of rugby they play, it's going to be a huge test for us," said Howley.
"It is the litmus test. If you're going to talk about being involved in World Cup finals and semi-finals then it should be something on the agenda.
"But on the back of that Grand Slam victory we're looking forward to it.This would be a greater achievement to win against the Southern Hemisphere sides away from home."
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