How many levels a B-tree has?

Hi,<BR>[I'm not sure if this will be the best word to express the thing I
mean, because I've been reading some papers, and mail archives that have
been looked for by this word, and I haven't got any result, so I would do my
best.]<BR><BR>I need to know (if it possible) how many LEVELS a B-tree index
has (to know how many accesses to the disk it would do in a query). I could
know the index size, but no-idea to find out how many&nbsp;levels it has.
I've read about pg_index, but there isn't any about.<BR>There is any way to
do it?<BR><BR>Thanks<BR><BR>Cris..<BR>