Short of scanning I don't think there's a absolute certain way to tell.
But watch for signs of season, response to back pressure etc
And keep her away from the boar - if she's not in-pig, she'll be desperate to get through to him and he'll be really interested in her
We AI'd our Berkshire today, and she was flirting with my OH before
Even led him into her ark for the 'deed' to occur - obviously shy about public displays of affection