Most of the House and Senate committee hearings take place in the House and Senate office buildings (e.g., Rayburn, Hart, etc.). There are no requirements to observe these hearings -- you just need to clear security to enter the building. Popular hearings will have queues and so it is possible that you won't be able to get into the room. Such queues created a mini-industry of line sitters that lobbyists and others employ to wait in line for them.

Attending a committee hearing is one of the lesser-known things I recomend to visitors, especially those who have been to the more obvious tourist places in D.C.