I have not heard anything as to why cranberry juice would be unsuitable for pigs with stones, other than that there's less point to trying cranberry juice with stone pigs than those with cystitis. Products/foods containing sugar is likely to irritate cystitis-like symptoms so I'd be more inclined to question whether cranberry juice is suitable for pigs with chronic cystitis issues.
Testing the urine can be done at home or by the vets, the vets can do a more in depth urinalysis though and can spin the urine to check for crystals. For a basic indicator, urine test strips (such as Multistix Reagant strips) will give you an idea of whether the urine contains blood, glucose, protein, nitrites and also pH and specific gravity levels.