On the other hand, it's a lot harder to tell if a female enjoys something than if a male does. At least, in my eyes.
It's all about body language, body language, body language, and I can't stress this enough... body language. With canines consent often comes from the non-verbal variety. So it is absolutely imperative that you learn their body language first, as this is the most effective way to communicate with you. As obviously they cannot give you a spoken "yes" or "no."
There are a few signs you'd want to look out for...
She is interested:
Usually flagging is the number one thing you'd want to look out for. This is where the female points her rear end towards you and parts her tail off to the side revealing her vulva to you. When a female canine is not in heat it's usually a little more of a subtle parting of the tail. However, when a female is in heat usually she will part her tail wildly. She may also lift her tail and swing it side to side.
Another sign you'd want to look out for is the female "aligning her tract." The vaginal tract is usually located 1-1.5 inches above the initial entrance of the vulva. What she'll do is lift her vulva up so that a male dog (or in this case a human) can easily enter her going straight in. She wishes to initiate sexual contact with you and is making it easy for the male to do so. Sometimes touching her just above her vulva will initiate this response, but other times she'll just do it when flagging and no touching is necessary.
Lastly, if you do decide she's inviting you for sexual contact and you decide to touch her, what is her response? She should be happy panting and backing herself up towards you. If you are actually inserting a finger you should feel an erect clitoris. (Which will feel like a semi hard lump just blow the entrance of the vagina itself.) You'll also want to make sure that you re not too big for her penis size wise.
She is not interested:
The most obvious sign is definitely her trying to pull away from you. She may also try to turn her head towards you, to come face-to-face with you. A more uncommon one that not everyone knows about, is what I call "step dancing" where the dog is lifting her hind legs in a stepping motion while you are initiating sexual contact with her. This is a sign that she's uncomfortable. If you initiate sexual contact and her first reaction is to sit down, she does not want said sexual contact. She may also tuck her tail tightly over her vulva and may even hunch her back while doing so. And lastly, excessive hollering, whining, and barking (it should never get to the level of "excessive".) There are more signs, but these are what I can think of from the top of my head, and are the most common to look out for.
If my doggy girl displays any of these negative signs, sexual contact stops immediately. There is no ifs, ands, or buts about it. You are to stop, and you are to pet her and let her know she has done nothing wrong by rejecting your advances. It's always important to let the dog know she absolutely has a choice in the matter. You should never try and "train" negative sexual body language responses out of her for your own gratification. Mutual pleasure is away the key.