Pick one up and look at it all over--head, breast, back, under the wings, and so forth. If it has feathers all over, it is fully feathered. Some chickens get their feathers much faster or slower than others, so I can't tell what age will be right for yours.
If you are not willing to put the heat source outside, then start by taking it away from them while they are inside. If they really peep and are unhappy, then they need the heat for a bit longer before you can take it away.
They need to change temperatures a bit graually, so first they have heat and are indoors, then you take away their heat but still have them indoors, then maybe later have them somewhere a bit cooler (like by opening a window where they are), then finally move them outdoors.
Also, you can take them outside for a bit during the daytime, and gradually have them outdoors for longer. So you might start with half an hour the first day (they probably have enough feathers for that already), and a little longer the next day, and in a week or two they might be outdoors all day long and only come in at night, and a bit later yet they would stay out all night as well.