Like reap said, there's no full-proof way to check - not by moving your body anyway because it's so easy to move your actual body instead. I'd say, if you think you're in SP, start envisioning what you want to dream about. Use dream incubation - call up the scenery. How it looks, how it smells, how it feels especially. Tactile sensations can be a really easy way to transition into a dream. Then just let the dream start forming around you. When you start feeling the wind in your hair and the grass below your feet (for example), then trying moving around and do a bunch of reality checks.