The ewe is in season for about 24-36 hours every 3 weeks. As we have pretty much only seasonal breeders over here, that is only during the period of July to January. If she's not a seasonal breeder can be any time of year.
It's said that the ewe may become a bit jittery and seem slightly unwell, but it's really only when there is a ram around that it becomes easy to tell. The sense of smell is key. When a ewe is approached by a ram, she will pee. The ram sniffs the pee, head then goes in the air, teeth shown as he assesses whether she is near to oestrus (Flehman response)
The ewe won't choose to be near a ram unless she is in season, but will go from 'cold fish' to 'brazen hussy' overnight!

. She'll stand by him, head down, presenting herself, following him around, sniffing his 'manhood' etc. For his part, he'll largely ignore her when not in season, but will track her down endlessly, jumping fences to reach ewes, staying with her, making low rumbling noises, licking her, pawing her etc. until she succumbs to his charms.
If there's just a fence between them, then the whole show will be played out across the fence and you could let the ewe go into the ram when she is receptive.