When Guru Har Krishan visited Delhi he stayed at Jai Singh's place ( which is now the site of Gurdwara Bangla Sahib). However it is said that Jai Singh and his wife (the queen) had some doubts probably because of Guru sahib's age( who was very young : 7-8 yrs of age at the time).
So in order to test the Guru that evening Raja Jai Singh sent his queen disguised as a maidservant.
After the evening prayer, the queen(disguised as a maidservant) visited the Guru, along with some of her maids, and asked the Guru if he could go with them to see the queen to discuss religion with her. The Guru said that there was no need to go to the queen's palace because she could hear him very well if he spoke from his seat. The maids again asked the Guru to accompany them to the queen's palace because it was the royal order. The Guru smiled and said, "I am sorry you care more for tricks than for truth."
Saying this he rose from his seat, picked up a small stick and walked towards the maids. He stroked the first with his stick and said, "God bless you." Then he stroked the second and said, "bhali karre kartaar" He stopped near the real queen, put his stick on her head and said, " Truth is more powerful than tricks. You need to learn more, your Majesty. The time is fast approaching when queens like you will become maids not in disguise but in reality." Everybody, was surprised at this incident and the Guru's words. The matter was reported to the king who felt satisfied that Har Krishan Sahib was the real Guru of the Sikhs.