Depends on the flux. Most will clean with Isopropyl Alcohol (IPA). This is more common in some regions than in others.
Many modern fluxes clean well with clean water. Most parts are designed to be waterproof, because a lot of manufacturers use water soluble flux. Some people wash boards with the dishes. The hard part here is residue from the water. In my shop, a final rinse with distilled water is required. Drying is another issue, but dry compressed air (static free of course) does that job.
Other fluxes may want acetone (from the paint store) but this is unusual. Beware, this is highly flammable and should be used outdoors only.