Hi TimHeather!
BarbaraH has given you some very good advice.
It sounds like your MIL may be in the denial phase. Or maybe she thinks just old people get alzheimers which couldn't be further from the truth. The sooner your FIL gets tested, the better off EVERYONE will be-no matter what the diagnosis.
Please don't worry too much. It's very possible your FIL doesn't have alzheimers. It could very well be something that's very easy to treat. One simple test your FIL or MIL might suggest to the doctor is a blood test for thyroid disease. I'm 41 years old and I have hypothyroidism. That can play tricks on the memory. There's no way of even guessing what might be wrong with your FIL. Call his nurse like BarbaraH suggested.
To answer your question about what tests are done to dx alzheimers-my dad had blood tests (I asked them to test for thyroid disease since it often runs in families). The doctor also did reflex tests, cognitive function tests. She asked a LOT of questions like 'Who's the president of the US, what day is it, etc.' The dx is probable alzheimers. Tomorrow, we go for the CT scan. I'm told that's to rule out any mini strokes or anything else that can cause his symptoms.
Let us know how it goes.
Good luck!
Barb