As Shortylicious has said, I think you need to consider your motivations before you take any action. If you just want to call her out on being a liar because it annoys you, I would think twice. This woman is a family member, and a close one at that. Trying to catch her in a lie in order to embarrass or shame her isn't going to help with relationships (yours, your husband's, and your daughter's).
What is the specific behavior you'd like to change? If you suspect that she is making promises knowing that she can't (or won't) keep them, then consider offering her an out up front. For instance, you mention that she isn't always up for going out or driving. I'm not sure what the reasons are, but if it is related to a health problem of some sort, she may not always know whether or not she'll be up for a trip on any given day. I'm willing to bet she'd be a bit embarrassed to call and explain that she's having horrible panic attacks or that her bladder infection means she can't be away from a toilet for more than five minutes. The bottom line is that if she says that she wants to visit you, you could say, "We'd love to have you, but I know that you have been having some ups and downs with your health lately. Would it be easier if we came to you?" Then, don't tell your daughter about the plans so she doesn't get disappointed if (when) they fall through.
If it's more of a desire to get her to stop groveling or otherwise calling you over and over to offer your excuses, then accept them the first time, clearly. "Oh, MIL, I'm so sorry to hear that your trick knee is acting up again. We'll miss you and we hope you feel better soon. Give us a call and let us know when you're feeling better." Kind of what Deetee suggests. Don't make it emotional or dramatic, treat it as though she is telling the truth. Make it clear that you are disappointed that she can't make it, but don't get across that you are disappointed in her. She isn't going to try to justify herself if she feels that she is forgiven. Hopefully, she'll eventually a.) stop calling over and over to reiterate how sorry she is and b.) realize that the reason why she can't make it isn't important (and therefore stop lying).