Try a 50/50 solution of vinegar and water for a streak-free clean. You can use a spray bottle or dip a rag in the solution, just like you would an ordinary cleaner. If you are worried about the fumes, only use vinegar for outdoor windows or mix a couple drops of essential oils to counteract the smell.

Mix equal parts white vinegar and water in a bucket (you can also make it in a spray bottle). Optional: add a few drops of dish soap to help cut through grease and grime. Use a dry microfiber cloth to remove dust or dirt from the window surfaces.

Combine two parts water with one part white vinegar in a spray bottle and you'll be all set with a homemade window cleaner. You can also combine warm water with a few drops of dishwashing soap if you prefer. Invest in a glass spray bottle to mix up your formula.

Check that they have the right insurance that covers them for the work that you'll be hiring them to do. Make sure they have the right safety equipment to safely clean all of your windows without posing a risk to themselves or members of your household.