I think the best features in the field are probably (not necessarily in order!):
1. Voice. But in most cases you do not hear them call.
2. Habitat. Lesser is unusual away from grassland or hillside habitats, Greater tends to prefer areas with some shrubs.
3. Size.
4. Mantle/wing colour. Lesser tends to be slightly paler/greyer in tone than Greater, which is usually very strongly rufous-chestnut.
5. Pale streaking on mantle/breast. Can be clearly visible sometimes on Lesser (see second photo of those posted here).
I've never really used iris colour in the field - partly because I forget there is a difference, but also because Lesser is rarely seen well enough or for long enough to check the iris colour.
Lesser has a very distinctive winter plumage (pale, streaked), which cannot be confused with Greater, but seems to be even shyer and harder to see in winter!
The birds here are Lessers. Ping Che is a very good site for this species.
