The best advice I can give is to learn the geography of the place you're flying over. This way, you should be able to learn where enemies are and narrow down where they might be so that when you approach the area, you have a smaller area to scan.
I know it sounds retarded but having come from flying jet sims where I had the privelage of a tarmac runway, I found IL-2 extremely frustrating because I had to learn how to work out what a grass airstrip looks like! It took me a while but I got there in the end and I found that similar practices can be applied to locating ground targets.
I hope this helps.
