I live abroad and took my boxer dog to the vet they took an xray and said he had cancer with out a blood test or swap sample how can they tell this from only an xray?
I'm sorry to hear this news about Minnie.
Some cancers are very obvious on xrays as they appear as solid masses where there shouldn't be one. Sometimes, especially in the pelvic area, they spread to the bone and form a very distinctive pattern in the bone that really can't be anything else.
Blood tests can be done but they tend to only lend evidence towards cancer rather than being definitive. Biopsies of the affected area are required for definitive diagnosis but sometimes these can be hard to obtain. In addition, taking a biopsy can cause a cancer to spread more than it would otherwise so it is not always adivseable to do so.
I hope this helps a bit and if you have any further questions please don't hesitate to ask.