The only sure way to see is to open the gear doors and this is not usually a routine requirement on pre flight/transit checks.
Therefore, a procedure whereby the gear bay doors are cycled should perhaps be considered? Or is this an impractical proposition? Could it, at the very least, be done on a regular basis when flights return to home base from places where stowaways have historically originated from?
A former ******** 747 skipper of my acquiantance, departing out of somewhere in Africa, received a message from ATC warning him of a possible stowaway, requesting he returned. As they'd been in the cruise for more than 1 hour, and was somewhere off the west coast of Africa, he found it impossible that the stowaway, if indeed onboard, would still be alive. So he reduced to max gear speed, cycled the gear, and continued on his way. Well, we were both drunk when he told the story, so could very well have been telling porkies. But what would you do in similar circumstances?