The three diagnostics, ranked
In order of how reliable each tell is when you have only one card in front of you: the drop shadow on the right side of the artwork, the 1st Edition stamp under the artwork, and the HP font weight. Use them in that order.
1. The drop shadow
Look at the right edge of the central artwork. On Unlimited prints there is a soft gray drop shadow running down the right side of the yellow frame. On Shadowless and 1st Edition prints there is no drop shadow. The artwork frame is flat against the yellow background. This is the single most reliable tell when you are looking at the back of a sleeve under fluorescent light at a card show.
2. The 1st Edition stamp
Look under the lower-left of the artwork. On 1st Edition prints there is a small black stamp that reads "Edition 1" inside a circular outline. Shadowless and Unlimited do not have this stamp. If the card has the stamp, it is 1st Edition. If it does not have the stamp but also lacks a drop shadow, it is Shadowless.
3. The HP font
On the upper-right of the card you will see the Pokémon HP. For Charizard it reads "120 HP." On Shadowless prints, the HP digits are visibly thinner and lighter than the Unlimited print. This is harder to use as a sole tell because the difference is subtle and bad photographs do not reproduce it well. Use it as a confirmation, not a primary diagnostic.
Why the prices diverge so much
Print run scarcity. 1st Edition ran for roughly two months in early 1999. Shadowless was a transitional print that ran for a few weeks after that, before Wizards moved to the Unlimited print that they continued to produce for years. Each step represents roughly a 10x to 30x difference in surviving population. The cost spread in PSA 10 reflects that.
A note on Jungle and Fossil
These sets did not produce a Shadowless variant. They went straight from 1st Edition to Unlimited. The diagnostic for these sets is simpler: 1st Edition stamp present or absent, plus the No Symbol error on early Jungle prints.