Initially, carry out a cross between the Hfr (high frequency recombination) and F‾, and the colonies that are ala+ strr are selected. If the ala region is donated late by the Hfr strain, then redo the cross but now the mating is interrupted early and selected for ala+. This selects for an F′, because this Hfr would not have transferred the ala gene early.
If the Hfr strain instead donates this ala region early, then use a Rec‾ recipient strain that cannot incorporate a fragment of the donor chromosome by recombination. Any ala+ colonies from the cross should then be used in a second mating to another ala‾ strain to see whether they can donate the ala gene easily, which would indicate that there is F′ ala. To differentiate the donor and recipient strains, it would also require another marker. For example, the ala‾ strain could be tetracycline resistant and selection would be for ala+ tetr.