You can. Basically this is what 3d pilots are very aware of. In full scale, sure we have instruments, but during private training (where you are learning stalls) you are actually encouraged to look outside and not inside. It's much easier to maintain heading by looking at a mountain in the distance, or a point in the sky (small cloud). Anyhow, there are various cues to look for, but the one that will be the easiest to "feel" in RC flying is mushy controls.

When flying straight and level, there is plenty of airflow over the control surfaces so the airplane feels nice and responsive. You're used to good roll rates, the elevator pitching the airplane up when commanded to, etc. As you get slower and slower, the airflow will diminish and you'll feel the stick start to get mushy. When you start to feel wings dropping, you are essentially at the buffet (right before a stall) and a stall is imminent. Just push the nose forward and you'll fly right out of it. Get used to these cues.