What early signs of Alzheimer's should I be on the lookout for?

Mom's been pretty forgetful lately, more than usual for her age. She misplaced her keys twice this week and forgot my birthday for the first time ever. I'm probably overreacting, but Alzheimer's runs in our family and I'm freaking out a bit. What are some early warning signs I should watch for? And how do I bring this up without scaring her?