Rather than using a mortar and pestle and a strainer, I use a garlic press wipe the juice straight from output of the press onto the glass surface of the refractometer.
The method that he recommends is very difficult to do at a market but I have found that using a garlic press you can check produce before you buy at a market.
As far as things other than the juice directly from fruits or vegetables, Di would be the best authority on that when she starts posting again.