the first thing you need to state is, what plane? and also what altitude are you flying at? its kind of difficult to tell how far you are from an airport using just your vision, as there is no real sense of depth in flight sim, its difficult enough in the real thing, it took me untill my third lesson just to learn where the 20 mile mark for the airport airspace was at bristol and thats using landmarks basically

As far as purely visual cues, unfortunately, Y2cragie is correct. Map mode, in views, can be a good reference, or you can use the distance rings on the GPS map view(with the range zoomed in). I'm assuming your starting with the 172/182, or something of similar performance. Also try using an airport that has a VASI(Visual Approach Slope Indicator). These are the red and white lights on the side of the runway. Not all airports have a visual lighting system like this, but using these for practice will help you establish a proper sight picture. If you  see two red lights, that is bad, you're too low. Obviously hitting the ground is bad, but seriously, a flat(low) approach limits you're forward visibility and may neccesitate the need for a lot of power. Two white lights means you're too high. This isn't quite as bad, as long as you have a long runway.  One red and one white light indicates the proper glidepath. To land at the edge of the runway "on the numbers", you should actually aim for a point just short of the runway.
                                     Good Luck!

You pretty much go by landmarks.   With the airport I fly from we have a highway a mile out, a mall 5 miles out, a reservior 10 miles out, etc.  Without using nav aids, you judge your distance using this.  How would you do it in IFR conditions?  Use nav freqs or the GPS.