She doesnt really pay much attention to me much anymore - Advice please

I have been with my partner for about 4 years now. I am 28 and she is 39, sex seems to have gone down a bit lately and she says its because she is tired.

She doesnt really pay much attention to me much anymore i dont think,i know she doesnt have time to be meeting other guys as she is always in the house with me or at work but when i met her 4 years ago she was with a partner.

So I was wondering how I could tell or what to do to find out if she is thinking about meeting other guys.

I think she loves me still but maybe she is not in love with me, maybe she thinks about other guys sexually, she tells me she hasnt masterbated for over a year, unless that is a lie because she only tells me that when i ask her to do sexual things,.

Im even tempted to set her up somehow to find out if thats what she wants.

Any help and advice would be great. Thank you