Personally I wouldn't crack any open to check fertility endless your selling them and don't want to hatch chicks, as each fertile egg could be a chick. And you will have some non fertile eggs in each batch usually, so save the fertile ones!
That's great that your shipped ones are hatching!!!
I successfully managed to candle some really dark brown croad langshan eggs and maran eggs recently. Firstly I used a powerful head torch (could use a normal torch) and a strong led reading lamp.
I got an opaque pot with a plastic lid, cut a hole in the lid so the torch light could shine...