Experience, mostly.

One of the easier things to try to spot in an upscale is an apparent blurriness or out-of-focus look to the image.

If the image contain lots of solid lines at any angles (other than straight up and straight across), you can look at them. Many posts at the original size will have nicely crisp, clean lines. Upscales will look blurry.

Other option: look for another version of the image for comparision. Though be careful or else you might wind up trying to upload a downscale.

I'd show you a sample here on Kona, but we delete upscales when we come across them.

Downscales are harder to pick out, but it's possible. We're a little more lenient on downscales as they tend to be less eye-cancerous looking than upscales.

I think one way I've been able to tell is to look at small details in an image and see if it looks more jagged than I would expect. Slight blurriness is also possible depending on the detail.