I wouldn't have a clue how to condition score them...
If you are interested in learning, visit the
Eblex Better Returns Nutrition and Forage page, scroll down and read 'Condition scoring of ewes'
Briefly, CS1- emaciated, C2 - lean, C3 - perfect, C4 - fat, C5 - way too fat
Basically, feel along her spine and at the base of her tail; if you can't feel the sticky-out bits on the tailbone, she's fat. If you can't feel them at all, she's very much too fat (CS5.) If you can feel them but only if you press really hard, she's too fat (CS4). If you can feel them if you press a little, she could be just right. (CS3)
On the spine, if you can't feel bones at all, she's way too fat (CS5). If you can feel the sticky-out sideways bits without having to press hard at all, she's probably about right (CS3). If you have to press hard, she's fat (CS4.)
Test on the loins, and behind the shoulders.
(We don't need to discuss anything less than CS3 for the sheep in that pic, they aren't thin

)
how do you stop sheep from eating?!
They'll eat what they need / want if it's available. So, if you have only one field and no subdivisions, then the usual option is to make them lamb every year - the work of growing and rearing lambs keeps them trim

And/or, if you know you have too much grass over the summer, then let them get thin - down to CS2, even - over winter, so that they have some leeway to gain flesh over the summer. Don't give them cake or much in the way of hay over the winter unless they start to become really thin.
The main worries about fat sheep getting pregnant are
- increased risk of getting stuck on their backs when they are full of lambs
- increased risk of prolapsing due to insufficient room for the growing lambs
- increased risk of difficult lambings
Some people also say that a fat ewe won't get in lamb in the first place, and then she just gets even fatter, and may get stuck on her back when she's got a heavy fleece on her.