This year has been a disappointment for investors.

The stock market is basically flat. Even if there's a "Santa Claus rally" to finish the year, stocks will be lucky to end 2015 up much more than 3%.

Most investors are already looking ahead to 2016.

Whether you're optimistic about the U.S. economy and stocks next year depends largely on one factor: Do you believe American consumers will keep spending -- or even pick up their spending?

"It's a consumer-led recovery. That is the key for 2016," says Andrew Milligan, head of global strategy at Standard Life Investments.

But problems abound. Beyond terrorism, there's China's slowdown, Greece's meltdown, the (arguably too) strong dollar, a manufacturing crisis and the energy sector on a lifeline as gas prices go back to $2 a gallon. And the Federal Reserve's rate hikes.

The question is whether Americans will shop enough to offset the headwinds. CNNMoney's Fear & Greed Index is sitting in neutral -- an indication that many are waiting to see what comes next.

Keep an eye on these three big warning signs:

1. Manufacturing is nearly in a recession

U.S. manufacturing is on the verge of a recession. It's been hit by a double whammy of foreign countries not doing so well (and thus not buying as much) and a very strong U.S. dollar which makes American products even more expensive when sold abroad.

"I see continued trouble ahead [for manufacturing]," says Alan Tonelson, who a manufacturing and trade expert who blogs at RealityChek. "I not only see no signs of major overseas headwinds flagging off at all, they could very well increase."