Then after you've mounted the tyre, pull it sideways so you can see down between the rim-edge and tyre. No tube should be showing. Then move over about 6" and pull the tyre sideways again and inspect the gap. Work your way all around the circumference of the tyre making sure no tube is visible. Then repeat for the other side of the rim/tyre.

Only once you've confirmed that no tube is caught between the tyre & rim, then you pump it up.