It is more or less impossible on a practical level to declare rulers to be Kuffar on the basis of whether or not they rule according to Shari’ah, let’s be honest.
Ruling is an action; it can either be a virtuous or sinful action depending upon its compliance with what Allah has revealed. But sinful actions do not eject someone from the religion. It is kufr less than kufr, as was understood by the earliest generations of this Ummah. With the exception of, for example, prohibiting Salah or criminalizing the fast of Ramadan, you are not likely to find a clear proof against a ruler that he has left Islam, unless you know him personally.
We should leave this line of thinking; it is extremely dangerous to our own selves
Furthermore, it is not even necessary. Those among us who zealously search for evidence that the rulers have become murtid, do so in order to incite their overthrow; but they are missing the point. The rulers are not the rulers; they are slaves delegated with the responsibility of imposing the rule of others. They are dictators who are dictated to. That is the nature of the Mulk Jabryy era.
You don’t have to search for way to make takfir on your actual rulers, because they are Kuffar. They are the global owners of capital, mostly Westerners, predominantly American. They dominate by means of a variety of instruments…the IMF, World Bank, their corporations, etc…There is nothing in the religion that restricts you from fighting them. You do not need to try to prove that your rulers are Kuffar, because the Kuffar are the rulers of your rulers; so direct your attention towards the real tyrants.