For the koshi-waza style uchi-mata, what distinguishes it from hane-goshi is the position of the leg (whether straight through uke's legs, or bent):
In the original uchi-mata, tori sweeps up uke's left leg with his right leg, which brings it under ashi-waza techniques. But increasingly, we see a kind of koshi-waza technique where tori loads uke onto his waist and sweeps him up.
#> Consequently, uchi-mata is classified under both ashi-waza and koshi-waza, but the koshi-waza type of uchi-mata described here is a practical technique.
This technique is similar to hane-goshi. If tori scoops uke up using the waist and leg, from a posture with his right knee bent, it is then classified as hane-goshi (photo 37).
Note the Kodokan New Japanese-English Dictionary of Judo makes allusion to the overlap between these two techniques:
haneru (to spring/jump) A situation in which the powerful effect of a hane goshi or uchimata sends your opponent's body high into the air.