Yes, you do need to be mindful of dehydration. Since your mom has Alzheimer's, she may forget to drink or not realize that she's thirsty. Her medications can also be
dehydrating; for example, blood pressure drugs could dry her out even if she does drink some water. And vomiting or having diarrhea for more than a day can cause dehydration in Alzheimer's patients, as it does with anyone else.
But simply saying, "Mom, drink something" may not help. She may not follow through. If you make it easier for her by handing her a glass, she's more likely to drink. (Watch her and remind her to sip it.) Fruit has a lot of water in it, too, so if she doesn't want to drink water or juice, offer an orange.
Signs of dehydration in anyone include being more confused or lethargic than usual. The person's sodium level goes up and confusion results (which also happens when sodium is decreased). But in someone with dementia, this change may be hard to notice. Still, watch for changes relative to how she usually behaves. If she seems even more mixed up or lethargic than usual, and if she also feels warm to the touch, alert your doctor. Sometimes people for whom this is a recurrent problem need to have an IV put in temporarily.