How do you know if the picture is High-Res?

"high" resolution depends on your frame of reference but in the case of moe a scan that will always stay has width and height at 1000+ pixels.

It's not all about resolution though, quality is an important factor too! This often factors in when images are slightly smaller than the previously mentioned guideline size.

scanning_resolution
In scans, you can only know if you know the prints.

Always take into account the image quality.