Jamieson, Fausset and Brown on Numbers 16:5-11

5-11. he spake unto Korah and unto all his company--They were first addressed, not only because they were a party headed by his own cousin and Moses might hope to have more influence in that quarter, but because they were stationed near the tabernacle; and especially because an expostulation was the more weighty coming from him who was a Levite himself, and who was excluded along with his family from the priesthood. But to bring the matter to an issue, he proposed a test which would afford a decisive evidence of the divine appointment.

Even to-morrow--literally, "in the morning," the usual time of meeting in the East for the settlement of public affairs.

the Lord will show who are his, ... even him whom he hath chosen will he cause to come near unto him--that is, will bear attestation to his ministry by some visible or miraculous token of His approval.