What's the best way to get to the player from a server script?

You would have to use a functions parameters to define a player, some kind of even must be triggered by one player to the server to be able to get the player. Examples of this:

game.Players.PlayerAdded:Connect(function(player)
    --code
end)

Part.Touched:Connect(function(player) --player is the player who touched the part
    --code
end)

Part.ClickDetector.MouseClick:Connect(function(player)
    --code
end)

--ect...
--FE:
--In local script:
local RemoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")

screengui.button.MouseButton1Click:Connect(function(player)
    RemoteEvent:FireServer(x) --x is defaulted to the second parameter when sent to server
end)

--In Server Script

local RemoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")

RemoteEvent.OnServerEvent:Connect(function(player, x) --Player is by defaul the first parameter, so x will become second parameter
    print(player.Name .. "Has pressed a button!")
end)

I hope this helped!
Comment if you need any help and if it work accept :D

The first parameter every function returns is the player.

So,

function FindPlayer(player)

end

Really the only way is to use function parameters, when the object is fired, it will also define the Player or Character that fired it.

game:GetService("Players").PlayerAdded:connect(function(player) -- returns player
    player.CharacterAdded:connect(function() -- returns character from player

This can be used with really any user triggers functions like a TouchedEvent

script.Parent.Touched:connect(function(whattouched) -- returns whatever touched the part
if whattouched.Parent:FindFirstChild("Humanoid") then -- continues by getting character
print(game.Players:GetPlayerFromCharacter(whattouched.Parent.Name)) -- prints player name from character