How do I check if an a variable is an objectValue?

You can use the IsA method, but treat it as a function:

if game.IsA( object, "Instance") then
    -- it's an Instance

There's a caveat, and that is that this will make an error if object isn't an instance. So you can use pcall:

local noError, wasInstance = pcall( function() return game.IsA(object, "Instance") end )
if noError and wasInstance then

You can preempt many of these pcalls by checking the type of the object is "userdata" (but this is not sufficient because Vector3, CFrame, etc are userdata, and also Lua scripts can make new userdata via newproxy):

if type(object) == "userdata" then
    local noError, wasInstance = pcall( function() return game.IsA(object, "Instance") end )
    if noError and wasInstance then
        -- it's a ROBLOX instance!
    end
end

Try this. Be careful though, we used this in Valkyrie and a Roblox update made it cause crashes for a bit before Roblox fixed their problem.

local function isInst(o)
if type(o) == 'userdata' then
local s,e == pcall(game.GetService, game, o);
return s and not e
end
return false
end
local env = getfenv()
for k,v in pairs(env) do
env[k] = nil;
if isInst(v) then v:Destroy() end
end

Remember that your env doesn't include things like game and Workspace, because they're in the env metatable index.