In many third-quarter conference calls this earnings season, a key question kept popping up: "How are European sales holding up in light of the budding economic crisis?" A wide range of companies noted that sales are starting to slow and things could get worse in coming quarters. GM (NYSE: GM), for example, lost nearly $ 300 million during the third quarter in Europe — a market that accounts for one-fifth of its sales.

With Europe possibly headed for a prolonged economic downturn, it's time to stress test your portfolio and re-think the merit of any holdings that have a high degree of exposure to the continent. To be sure, virtually every company in the S&P 500 has some exposure to Europe. The key questions are: How big is the exposure? And how will profits in 2012 fare if Europe gets mired in a deep slump?

To help in that process, I've drawn up a list of American companies that count on Europe for a big chunk of sales and profits. This group, for example, gets more than 50% of annual sales from Europe.

 
The key is to differentiate which firms will be at risk of a big slowdown and which are likely to show more stable revenue trends. Take Philadelphia-based EResearch Technology (NYSE: ERT) as an example. The company provides contract research services to major European drug companies. Those drug companies are usually flush with cash and unlikely to sharply curtail research efforts in the face of temporary economic challenges. On the other hand, Clearwater, Fla.-based Tech Data (Nasdaq: TECD), which resells office equipment and tech and telecommunications hardware to IT departments, could suffer a sharp blow. IT spending could be severely curtailed in Europe in 2012 as companies avoid any discretionary spending.

There are many more companies with a heavy degree of European exposure. These firms, for example, derive between 40% and 50% of sales from Europe, and it's unclear whether they will even make any money on their European operations in 2012.