Should Monarchs be Honored?

According to Judaism, should monarchs be respected and why? How are we to know if they are G-dly? Why is there a blessing for them?

Definitely! Our rabbis stress that Ya'akov Avinu & Moshe Rabbenu, correctly honored Pharoh. As to why, firstly, if they really are powerful- it's wise and beneficial to be on their "good side"! Secondly, the blessing which we make says explicitly that God is the One Who gave them their position and power- so it's also respecting Him. Thirdly, Rav Kook (Ain Aya Brachot 58a) explains that political leadership, even though often misused, brings order & prevents anarchy. Fourthly, it says in the Talmud (Brachot 9b) that we should educate ourselves to appreciate, when the day will come, the difference there will be in respecting the Mashiach, the Jewish king and his truly deserved honor and our eternal & spiritual national honor as the Chosen People and Light to the Nations- as opposed to the temporary, relatively superficial honor of other kings and their external, sometimes even absurd honor (see Stalin & North Korea's Kim Jong-un). That's one of the reasons why there's a different blessing for a Jewish king. Accordingly, and fifth of all, it's important to remember and stress that our nationalism is Jewish/Am Yisrael, and not American nor English, and with all true & due respect, the Queen is not ours but "theirs".