http://www.lua.org/pil/20.2.html
As we can see, we can't just use string.find(var, ".") because .find is meant to find patterns in strings and not strings within strings. That's why there is "%d", "%w" etc. And, "." matches all characters according to the link above.
Since we're looking for a pattern, we need to use the pattern operator before what we want to find (%). So string.find(var, "%.") would work. From this we can also see string.find has an argument on the end of it called plain. That matches a plain character without any pattern operators (so I can search for just "w" instead of "%w" which matches alphanumeric characters. So, you can do string.find(var, ".", 1, true). 1 represents the string index to begin at.
tl;dr
var = "100.000.000"
string.find(var, "%.")
-- or
string.find(var, ".", 1, true)