Here's the real scoop, if you're actually fucking.
Firmly place your hand right below their neckline and judge the reaction. The biggest thing you're looking for is any set of reactions that show Excitment, then Anticipation. Sharp intake of breath, followed by eye contact (Or the look back, if you're hitting from behind) is almost always a go-sign. To confirm (And as a tease), slide your hand up the side of her neck, keeping your thumb OFF the neck. If she's into it, she'll probably push her neck into your hand, if not she'll push off (In which case, trace your hand down the shoulder as a saving move) . If still good, then you bring the thumb down, so you are now gently grabbing her throat. Not choking, but a definite grasp. At this point, there is clearly no ambiguity, and you've taken metered steps to allow her any realistic chance to opt out. If she suddenly doesn't like it, abort, but be confident that you did the right things. If grasp was successful, gradually add pressure until she indicates that you should stop or until you are uncomfortable with how safely you can do it.
If at any point she shows indifference to the play, don't go for it. If she's not paying attention to your hand, then she's not anticipating what you're doing with it, so she probably doesn't know if she'd like it. After sex, ask her directly. Don't ever ask during sex. Despite the necessity of consent, questions are absolute mood killers in the actual moment.
Its also not a good idea to lead with the move, cause bitches love subtlety and that ain't subtle. Try to transition onto the neckline from somewhere else, like their face, shoulder, or hair. Following the touch of your hand can be a huge part of sex for the receiver, because they can focus on multiple points of contact due to not thrusting. Give them something to immerse themselves in the act, and they'll enjoy it far more, and be more receptive to trying new things (and having sex more often).