Unlike transits of the sun involving the planet Mercury, those of Venus are readily visible with the unaided eye; the planet appears as a distinct — albeit tiny — black spot with a diameter just 1/32 that of the sun. This size is large enough to readily perceive with the naked eye. [Venus Transit of 2004: 51 Amazing Photos]
But Again . . . Be Careful!
Eye safety is always a prime concern when dealing with the sun. Observing a transit is a lot like studying sunspotsbecause, after all, you are looking at a dark spot on the sun.
But trying to see a transit is also like trying to view a solar eclipse. You have to be ready at a particular time, and you may have to travel far from home. For the transit of Venus, however, your exact location is much less critical than it is for a total solar eclipse.
In particular, observers in Eastern North America, where the transit will happen in the early evening, your observing site should have a low horizon to the east-northeast. It is a good precaution to check the sun's setting point, to verify that trees or buildings do not block your view. As Venus moves across the face of the sun, it will appear absolutely jet black in contrast to the lighter gray of any sunspots that may also be present on the solar disk.