You can only use S11 measurement. I think Balanis book has a nice and
simple but still accurate way to calculate antenna efficiency through
measurement.
Assuming that your antenna is small enough (say biggest dimension
not more than 10 cm or so) idea goes like this: In open space Q factor
includes radiation, resistance, dielectric losses. When you cover(!) the
antenna completely within a shielded metallic box, then radiation losses
dissapear and Q factor increases. There is a formula that connects Q factor
with S11 parameter. So, it's possible to relate power in open and shilded
enviroinment and hence extract radiation efficiency.
I've seen several researchers using the above procedure to get approximate
but quick results on antenna efficiency! Nice...