  The use of parameters measuring order-parameter fluctuations (OPF) has been
encouraged by the recent results reported in \cite{RS} which show that two of
these parameters, $G$ and $G_c$, take universal values in the $\lim_{T\to 0}$.
In this paper we present a detailed study of parameters measuring OPF for two
mean-field models with and without time-reversal symmetry which exhibit
different patterns of replica symmetry breaking below the transition: the
Sherrington-Kirkpatrick model with and without a field and the Ising p-spin
glass (p=3). We give numerical results and analyze the consequences which
replica equivalence imposes on these models in the infinite volume. We give
evidence for the transition in each system and discuss the character of
finite-size effects. Furthermore, a comparative study between this new family
of parameters and the usual Binder cumulant analysis shows what kind of new
information can be extracted from the finite $T$ behavior of these quantities.
The two main outcomes of this work are: 1) Parameters measuring OPF give better
estimates than the Binder cumulant for $T_c$ and even for very small systems
they give evidence for the transition. 2) For systems with no time-reversal
symmetry, parameters defined in terms of connected quantities are the proper
ones to look at.
