The best indication is a second HCG blood test 48 hours apart to check the levels have risen, and how much.
Also, policies may be different at different hospitals, but I know when I’ve had early scans (i have had many over the years) I’ve been told levels of around 2000 are when you see things on a scan. But it could be that you only needed one more day to see something, so I would try not to worry even though it’s hard. There are Massive changes this early on even a few days can make a difference. And Definitely get bloods done.