why are you going to Langkawi to spot?! surely there are better places to spot ; and indeed there are better things to do in Langkawi ...
But if you have to - LGK has a small but rather nice and neat terminal (without aerobridges, so you walk right up to the plane - that's nice
) and is served by three airlines: MAS (737-400 up to about 10x daily, A330-300 on Tue/Wed/Sat), Air Asia (737-300 1x daily) and Silk Air (A320 or A319 1x daily). During the northern winter only, you also get a Finnair MD11 charter weekly. In the past LTU also ran a winter 767 service but that has since terminated.
I know that if you drive from the airport to Kuah Town, there is a scenic point where you can park your car from where you have a commanding bird eye's view of the airport and runway.