At how far along should a doctor be able to tell that a overweight woman is pregnant by giving her an abdominal check up?

It depends how overweight she is. If only a little overwieght it shouldn't make much different, but if extremely obese it will be impossible to tell by abdominal exam. Over 30 years ago I was a student nurse training in a pre-natal clinic and an extremely obese woman came in thinking she was about 28 weeks pregnant. I couldn't feel anything and nor could the trained midwife or the ob/gyn and an ultrsound scan confirmed that she was not pregnant. Women at both ends of the weight scale can have their periods stop.