Good Question!  How you evaluate the use of a discovery, or hands-on
room, can vary depending on the goals of the facility.  All too often, this
type of evaluation is conducted by simply asking "what did you learn?" to
a group apon their departure. This type of question, when directed at
families and school groups, assumes that the facility is to be a place
where  formal education takes place.  The goal in this type of
environment is the expression of a set of ideas and their retention by
visitors.   It is similar to a traditional student-teacher classroom
environment.

  I think a better suited approach is to look at the discovery room as a
place of "discovery" or independent learning.  Visitors may not remember
specific facts at all after their visit.  They may explore, investigate, and
walk away believing that even learning can be fun (which in my opinion
is far superior to the memorization of facts).

A way to evaluate sections of the room could simply be to observe their
use.  Which areas are popular?  Which are seldom used?   Volunteers
and staff are usually well aware of traffic patterns.  Maybe some areas
of the room are more popular because visitors believe they are "more
fun".  Perhaps amusement and enjoyment determine which areas and
activities are favorites. ( Note that these areas may not also be the most
educational as well).   Questionaires and exit interviews are a good way
to solicit responses from families and teachers as well.  They may be
able to describe which activities the children liked as well as which were
conducive to independent learning.

These are just a few ideas.  Good luck.