This has been a big problem for most of our 23 years together. She has done almost nothing to let me know when she is interested. That has left me to guess and more than a small number of refusals.
Things have gotten a little better because we have a routine of Friday & Saturday night - but trying to get a little on a weekday is still a hit-or-miss proposition.
Obviously communication is important - it get's you what you want and limits frustration for your partner.