Keep an eye on price-to-earnings multiples for airline stocks, according to TheStreet's Action Alerts PLUS Portfolio Manager Jim Cramer. Cramer said Alaska Air (ALK) - Get Alaska Air Group, Inc. Report and Southwest Airlines (LUV) - Get Southwest Airlines Co. Report are seeing their price-to-earnings multiples increase. "They're now at 12 - that gives an umbrella to the companies that were selling at just 6 and 5 times times price-to-earnings multiples, that are now at 7, 8 and 9," Cramer said. "I'm talking about American Airlines (AAL) - Get American Airlines Group, Inc. Report , Delta Air Lines (DAL) - Get Delta Air Lines, Inc. Report and United Continental Holdings (UAL) - Get United Airlines Holdings, Inc. Report ." Cramer spoke earlier from the floor of the New York Stock Exchange.