In the past I have accomplished this by looping on the list and setting a flag if I encounter the desired obj type.

flag = 0
for(var/obj/typeX/X in LIST)
flag = 1

// blah blah

However, this seems pretty inefficient, so I'm looking for an easier method. I tried the following, but I get a compile error (error:var/obj/typeX/X:undefined var):

if(var/obj/typeX/X in LIST)
// blah blah

What is the best way to accomplish this?

if(locate(/obj/blah) in blahlist)
This will check to see if a blah object is in the blahlist.