Spec notwithstanding, DVI can go much longer than 18'. With 720p and 768p signals I know of a few dozen people who are successfully using standard copper cables at 30 feet, and a couple who have gone up to 50'.

At 10 meters, slightly more than 30', I have personally seen 1080p transmitted. You would probably start to see degradation at 1080p at 50' though.

Signal problems are very easy to spot in DVi transmission. They appear as single pixel bit errors, usually called "sparklies". They appear as single pixels that are dramatically the wrong color, i.e. a light blue pixel in a dark red error or some such.

When your DVI cable is on the verge of being unable to support your signal, you will see these. When it is a bit worse, you will simply lose picture entirely.

BB