«وَإِنْ كُنْتُمْ فِي رَيْبٍ»
And if you are in doubt.
Kuntum is a past form, yet the verse addresses people whose doubt can be present when the challenge is made. The condition is therefore not restricted to the beginning of doubt at some future moment.
The rhetorical interpretation must consider the context of challenge and the actual audience. The particle frames the addressees' state as a condition for an argument, not as an admission that the speaker does not know whether doubt exists.