How to find if table have table in it with given name

How to find if table have other table in it with specified name?
Or how to conver table "var1" in table "t" into string? And use "for" loop with "if" to find it.

input = "var1"

local t = {
    var1 = {[..content..]},
    var2 = {[..content..]},
    var3 = {[..content..]},
};

Now how to find, if table "t" have table "var1"?

>Solution :

Extending on Mikes comment: indexing a table with a non existing key returns nil. See https://www.lua.org/pil/2.5.html

And since nil evaluates to false checking for existing keys is as simple as

if t["var1"] then

Or when the identifier is valid:

if t.var1 then

If you want to make sure var1 is a table only:

if type(t.var1) == "table" then

See https://www.lua.org/pil/2.html