You can tell a male hammie by the width of the base of its tail. The males is wider due to its testicles being there.
Here is a picture on this site with more info to help you understand what I mean.
Hamster Sexing
I have rescued many a little one that was bred willy nilly by folks and then knew nothing about the care or by people who bred and than ended up deciding that the whole baby thing was just overwhelming.
Are they Dwarf hamsters? Or Syrian Hamsters?
For both they they will get really fat towards the back of them and it will happen all of a sudden. This however does not happen until the last part of the pregnancy She will also start eating a lot more (If it turns out she is she should have extra protein so you may want to add some just in case. Things like cheese little bit, nuts peanut butter and oats are good I used those rolled oats they have for birds I also have gotten steel-cut oats in the organic and natural food aisles of grocery stores.
If she is pregnant she will build a really large nest and hoard food Mind you hamsters do that a lot anyway but this will be a bot different in the appearance of the nest
In most cases I have seen the mom will deliver 2 to 7 days after building the nest and gaining the weight.
So basically if you do not get any babies in the nest 10 to 13 days you likely do not have a pregnant hammie.
If you do find she is squatting and running around a lot she may be delivering. She may have them in the nest she may have a few out of nest this is OK do not interfere she will go back and gather them into the nest. You do want to limit holding her she may be a bot nippy and grouchy when she is getting close to having them. So limiting holding if it turns to be the case
Be aware they grow FAST so it is a good idea to get homes lined up placing ads when born is best.
Also splitting males from females as soon as they are eating on their own is a must or you may be passing on pregnant ones as well. Many people make mistakes on sexing and end up doing just that These guys can start breeding really young so just make sure to check that picture
Here is a good site that tells all about Pregnant hamsters Just in case
pregnant
The site I linked at the top for sexing also offers great info for general care as well as a message board.
Hamsterific forum
Good Luck Hope this helps