Possible Vac Leak?

I have a 1991 Mercury Grand Marquis. I was on a long drive ( about 2 hours ) and all of the sudden the engine started to spit and sputter and run like crap. It seems to have a hard time going up hills and stuff. So far I've replaced the fuel filter spark plugs, & Cap. Could this be a vacuum leak? also the window washer fluid is full but does not shoot out. ive heard this could be a sign of a vacuum issue. how do i check for that? I know about spraying starter fluid but I am not about to spray an explosive liquid near spark plugs. any info would help. thanks!