As Guros said, the command would be for example "/list Staryu" or "/list Route 41".
Where this falls short, is it doesn't tell you where you should actually hunt for a given pokemon. You would have to type /list for each map where Staryu appears to find the map(s) where it's available at the lowest (easiest) rarity tier.
@AsianKid has put in humungous efforts lately updating the Wiki, including rarity tiers. I'm not positive how far he got but I think you can rely on it for the most part, and double check in-game with /list if necessary. You can see here
https://pwo-wiki.info/index.php/Staryu that Staryu should be easiest to catch in Johto Safari Entryway or Seaside - tier #2 on both maps.
One shortfall of the Wiki is it doesn't always list Pokemon that were featured in past events/swarms.
I have created a list of those which is a valuable resource if you want to build wealth in PWO or trade knowledgeably. You should know for example that Sneasel was Tier 1 during a semi-recent Christmas event so it's a complete waste of time to hunt it at Tier 5 (easiest currently-available tier). There are so many Sneasels left over from that event, they're literally worthless unless they're shiny or have amazing IVs. This document also has the 'current' spawn list which you can search in, which might be easier than using the Wiki.
You should also note, if two maps both have something catchable at "Tier 2" for example the actual difficulty can be quite different on those two maps. The only way to know/guess is by asking other players or by trial and error. This is due to PWO's obsession with "Lore" which you can interpret however you want lol.