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.