If it appears that the server is tossing the ball in an upward direction before striking it ,then the serve is"legal" The idea of "teeing' the ball off of your non serving hand is a very tough subjective call to make. If the motion appears to be as if the server is throwing the ball over the net with both hands, then that serving motion is a violation of the rule. Again, I always revert back to the need for a ref in a tournament setting. Their job is to make those determinations and if they are not comfortable with that ,then go to the T.D. and ask for assistance. In a pick up game, making that call on you opponent, will usually lead to conflict. "Go on to the next play". Adapt, adjust, anticipate, accept the challenge.
You can bring it to their awareness, but in reality you can't make them change it. JUST GET READY TO PASS THAT BALL.
Know that it will come over the net with a different trajectory and require more concentration.
The game of volleyball has many subjective rules and guidelines and the nature of the way the ball is handled will always be subject to personal style. The real problem is when that personal "touch" creates a definite advantage for a player. I guess you could not choose to play against that opponent and with the right feedback from the volleyball community, that player would "get the message" and make the needed changes.
I approach these questions from years of running and participating sanctioned events and have accepted that these types plays that happen over the duration of a tournament have a way of balancing out. The Volleyball GODS are watching....

mhullo