The European and US intelligence services estimated the death toll at ~50 and 100-300 respectively.

I assume satellite imagery and counting...there's probably also UAVs from all the great powers recording footage.

Seperately im fairly certain the Israeli security services have spies all over gaza. Somewhat grimly I think the air strikes have had a large component of people on the ground marking areas for strikes after confirming the presence of some Hamas member. It was a strategy exploited during the Russia/Ukraine war and it's something Israelis can do because they are largely the same ethnicity.

As for the size of the explosion it's not a great source but you can review pyrotechnic demonstrations of actual military munitions versus like a small special effects explosion and a lot of fuel. The former tends to not actually have a huge fire ploom like the hospital bombing had. There is one it's just relatively small. If the rocket misfired and had a lot of ignited fuel spraying over the area... It causes that sort of imagery and casualties.