In addition to other tools, there's also https://bgp.he.net which has a page for announced prefixes. Sometimes it's useful to use multiple tools, as not everyone will see all announcements or process them the same.
At the moment, https://bgp.tools/ is my favourite goto for ASN/BGP-related information. They operate route collectors on quite a few internet exchanges, collecting BGP routing information from peers connected on those IXes, as well as route servers. They offer near-realtime insight in what prefixes are actually announced/visible, as well as prefixes that have been allocated, but are not announced.
There's even more under the hood, worth checking it out.
There's even more under the hood, worth checking it out.