Comparing observing logs for faint objects - or objects with faint or elusive features in them - requires us to also have a yardstick with which to compare conditions under which each observation was logged. Without this, a nebula may be "easy" for one observer, but inexplicably "very difficult" for another, with no apparent reason why.
To resolve this, we ask observers to estimate their sky
conditions - and in particular, how dark their sky is -
when entering an observing log in IAAC. There are a variety
of more or less quantitative, objective methods for doing
this estimation. Many of them are summarized in this paper
entitled "Visual Estimations of Night Sky Brightness", from
the George Wright Forum, by Mr. Chadwick A. Moore:
http://www.georgewright.org/184moore.pdf
Limiting magnitude is a very common technique, as it just
involves counting stars with the unaied eye in one of 30
different star-delimited regions around the sky. This page
at the International Meteor Organization website summarizes
the method, and has links to easy-to-use star charts for
identifying all but the most southerly of these regions:
http://www.imo.net/visual/major/observation/lm
Telescopic Limiting Magnitude can also be estimated, by
counting or searching for faint stars right at your eye
piece. This technique requires some practice, but it is
summarized in the following paper:
http://members.csolutions.net/fisherka/astronote/plan/tlmnelm/LimitMagFields.htm
Long-time comet observer John Bortle has also developed
a very useful, somewhat objective way of estimating the
overall darkness of the sky on a 1-9 scale in the area
where your telescope is pointing. It is summarized here:
http://www.skyandtelescope.com/resources/darksky/3304011.html