In theory you can ultrasound or xray a hamster but this is not really necessary. A vet would be able to palpate the hamsters abdomen and give an idea of whether the hamster is pregnant or not. As the pregnancy progresses the hamster will become larger and start to 'look pregnant'. Hamsters are solitary animals and for a hamster to be suspicious of being pregnant then they need to have been introduced to a male. The gestation length is only 16-18days in hamsters and so usually the suspicion will tie in with this period. If you do suspect your hamster is pregnant or, if you suspect another cause of the hamster getting fatter then a check over by a vet is recommended
By Hannah James-Roll BVSc MRCVS (Professional Advisor)