OFDM transmitter data rate
A bit rate can't be "measured" like a physical quantity but it can be calculated from the following parameters (given the subsystems you mentioned):


$f_\mathrm{S}$ sampling rate
$N_\mathrm{SC}$ number of subcarriers (= IFFT size)
$N_\mathrm{U}$ number subcarriers used for data transmission (excluding possible pilot tones)
$N_\mathrm{G}$ length of cyclic prefic in samples
$M$ number of bits per subcarrier (for L-QAM, $M = log_2L$) (*)
$r$ coderate of FEC, where $r = k/n$, with $k$ length of code word, $n$ length of information word


The length $T$ of an OFDM symbol is the basic symbol length plus the length of cyclic prefix:
$$
T = \frac{N_\mathrm{SC}}{f_\mathrm{S}} + \frac{N_\mathrm{G}}{f_\mathrm{S}}
$$
Each OFDM symbol contains $N_\mathrm{U}$ subcarriers that carry $M$ bit of information each. Thus the bit rate $R$ is (excluding FEC):
$$
R = \frac{N_\mathrm{U}M}{T} = \frac{N_\mathrm{U}Mf_\mathrm{S}}{N_\mathrm{SC} + N_\mathrm{G}}
$$
Finally, if some overhead by FEC is added, the net bit rate $\tilde{R}$ is given by
$$
\tilde{R} = rR
$$



(*) A note on $M$: as Jim Clay has pointed out in his comment, the number of bits doesn't have to be the same for all subcarriers but is sometimes chosen individually for each subcarrier. This is often refered to as "bit loading". In this case $N_\mathrm{U}M$ in the expression for $R$ has to be replaced by
$$
\sum_i^{N_\mathrm{U}}M_i
$$
where $M_i$ is the number of bits on the $i$th subcarrier.