If you have a male, he will tell you.
I mean, unless the dogs are loose all the time together, and you do not supervise at all. Because he will begin to show a lot more interest in her, and will "groom" her, lie by her, and so forth.
This is the point where he might mark in the house, because if she drips, he will lick it, and then he may want to put his scent on the area to let other dogs know that he is the Dog when it comes to this bitch.
And, long before he makes his first attempt at mating with her, he will be checking her urine, and grooming her, face, ears, pieces parts. And then, she will usually refuse for a while. He will try to get her to stand for him, and she will lie down, run away, and so forth. Somewhere, usually in the middle of the cycle there is a 3-day period where she is in "standing heat" This is when she is willing to stand, flagging her tail, the dog is interested, and puppies are most likely to result.
Then if the male is experienced, he will provide his sperm.
If the male is not experienced, he will jump onto her, sometimes on her head, and sometimes he jumps off way to soon. Often the moment he starts getting close, she will swing around growling, yelping, or snapping at him, and he may be deterred. And maybe not.
But the old oops litter -- maybe, if you leave them together unsupervised through the heat cycle, but that isn't an oops, that is deliberate breeding. Otherwise, maybe if you are taking mood altering substances you won't notice what is going on. Maybe.
It really isn't like going to the dog park, not realizing your bitch is in heat, and you turn your head to see a new dog come in, and you look back and your bitch is tied with some dog -- nope, it takes time and is very obvious.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.