It is usually easier to tell the right side before it is washed by the way it was folded on the bolt. If in doubt I will put a mark on the selvedge down the fabric to help me remenber.
Often the best way is to look at the weave in the selvedge. The right side will have a more finished weave then the wrong side.
If neither of this works then just mark to use the same side throughout the piece in case some difference should show up in use.
and stilll if there is not difference in look I just try and use the same side through the work.