A pair of Hotline polarising filters and a light source (e.g a lightbox / torch) are all you need to detect stress in glass.
Hold one in front of a light, then the other around 2" further forward at 90 degrees to the first. Hold your piece of glass in between the filters, any stress inherent in the glass will show up as a white halo effect (see picture).
Why not use these on test strips prior to going ahead with your project - could save costly breakages!
We always reccommend that you use a tested compatible glass, e.g. the System 96 range, but don't mix your glass types!