The office of the Grand Ayatullah Khamenei (may Allah prolong his life):
A1) All followers of the misguided and deviated Baha’i faith are najis and all transacting and interaction with them must be refrained from.
A2) It is not problematic per se.
A3) It suffices and there is no need to investigate.
The office of the Grand Ayatullah Makarem Shirazi (may Allah prolong his life):
1 and 3- Transacting with advocates of the misguided Baha’i sect is impermissible, and if someone is known to be a Baha’i, that is enough to prove his Baha’ism.
2- In the case of the question, it is permissible.
Ayatullah Mahdi Hadavi Tehrani (may Allah prolong his life):
1- Although transacting with the disbelievers, including the Baha’is, is permissible per se, but since this misguided and deviating sect is a supporter and companion of Zionism in its theoretical opposition to Islam, any form of aid to them is haram and must be refrained from.
2- In the case mentioned, it is permissible to give the clothes to the dry cleaner’s, although renting that place from the Baha’i by the Muslim is haram in the first place because it falls under the category of collaborating with the Baha’is.
3- If he says he isn't, that will suffice, unless it is proven otherwise, and there is no need for investigation.