  We present an analytical method to estimate the condensate fraction $n_0/n$
in strongly correlated systems for which the zero-temperature static structure
factor $S({\bf p})$ is known. The advantage of the proposed method is that it
allows one to predict the long-range behavior of the one-body density matrix
(i) in macroscopic and mesoscopic systems, (ii) in three- and two-dimensional
geometry, (iii) at zero and low finite temperature, and (iv) in weakly and
strongly correlated regimes. Our method is tested against exact values obtained
with various quantum Monte Carlo methods in a number of strongly correlated
systems showing an excellent agreement. The proposed technique is also useful
in numerical simulations as it allows one to extrapolate the condensate
fraction to the thermodynamic limit for particle numbers as small as tens to
hundreds. Our method is especially valuable for extracting the condensate
fraction from the experimentally measured static structure factor $S({\bf p})$,
thus providing a new simple alternative technique for the estimation of
$n_0/n$. We analyze available experimental data for $S({\bf p})$ of superfluid
helium and find an excellent agreement with the experimental value of $n_0/n$.
