If you have white paper based bedding you'll usually be able to see soiled bedding in one of the corners. To encourage him you can put some of the soiled bedding in the sandbath, and you'll know if your hamster has used his sandbath as a toilet if the sand "clumps up".
)
Some hamsters even likes to have an exclusive sandbath for the toilet and another one for bathing.
Usually I just take a chunk of the bedding in one of the corners and smell to it. If it smells like pee then I'll throw it out and replace it with new bedding. There is really no other way to know other than through smell.
And congrats on your new hammy!