How to tell which old cards can deal damage to planeswalkers
The Dominaria Frame, Template, and Rules Changes article has the set of rules they used to decide which cards to errata (under the "Planeswalker Redirection Rule" section).  To summarize:


"target creature or player" --> "any target" (ex. Lightning Bolt)
"target player"/"target opponent" --> "target player or planeswalker" (ex. Lava Spike and Jeskai Charm).  Exception: the text is unchanged if the amount of damage is calculated by using information about that player or objects they control (ex. Sudden Impact or Jovial Evil)
Everything else has not changed, except for Vial Smasher the Fierce, which can optionally damage any planeswalker that player controls.


Spells that deal damage to a player and then do something else to at least one permanent that player controls (like Bonfire of the Damned) have been additionally patched: “that player controls” &rarr; “that player or that planeswalker’s controller controls”. 

These rules do not apply to cards printed in or after Dominaria. For example, Gravitic Punch cannot target planeswalkers as it was printed post-errata. You can tell that a card was printed before Dominaria if it meets one of these criteria:


It has a copyright date of 2017 or earlier.
It was from the Rivals of Ixalan set, which has the set code RIX and this set symbol: 
It was from the Masters 25 set, which has the set code A25 and this set symbol: