We know that dogs are highly intuitive, and incredible aware of our behaviors and emotions as humans. Humans and dogs share a unique bond, thanks in part to oxytocin, the "bonding chemical" we release when gazing at or cuddling a loved one. Dogs are aware that we love them, but little is known about whether they can sense their owner being in love with another human. There is virtually no research on the subject.
Though it certainly seems sometimes that dogs know exactly how we're feeling, there's no definitive proof that dogs can understand human emotions. Humans have the tendency to project their own feelings onto their dogs, and to assume dogs have the full spectrum of emotions we humans feel — or that they can at least understand them. However, we don't have any proof that that's the case, so it's tough to say whether a dog would understand when their human was in love.
However, if you're in love, you're probably in high spirits, and that's likely to benefit your dog in ways she will notice. You might be inclined to go for more walks, or to spend more time at the park with your dog. Basically, she'll notice if she directly benefits from you being in love.
Though your first guess may be "cats don't know or care how we're feeling," cats aren't as uninterested in us as they sometimes seem. A 2015 study suggests that cats can read their owners' facial expressions. The cats in the study were more likely to display "positive" behaviors, such as purring, when their owner was smiling rather than frowning. This research suggests that cats can understand some human displays of emotion. However, we don't know how much they can understand beyond the very basic expressions of smiling and frowning.
Cats, like dogs, don't have any concept of romantic love, so it's unlikely that they would understand when their owner was smitten with another human. That said, there is almost no research into the subject, so we can't say for sure.
Also like dogs, cats may notice that they are reaping the benefits of your being in love, without being aware of the cause. You may be in a happier mood, leading to more petting and more playtime. Your cat might notice that you're being extra fun, and she probably won't care why.