It is largely recognized that gamma-ray burst (GRB) jets involve ultrarelativistic motion. However, the value of the Lorentz factor G0 is still not clear and only lower limits are known for most bursts. We suggest here a new method to obtain upper limits on G0. The early high-energy synchrotron afterglow flux depends strongly on G0. Upper limits on GeV emission therefore provide upper limits on G0. Applying this method to 190 Fermi GRBs which have not been detected by the Fermi-LAT, we place upper limits on the high-energy afterglow flux, and in turn on G0. For bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of the order of a few hundred.