That's why they make SNMP tools. But short of that, it’s actually not that easy. For most part, you have to discover them implicitly, by being a bit of a detective.
In fact, before even delving into the problem, what do YOU mean by router? As in, something you're actively routing to, or simply something hanging out on your network but otherwise unaware of? And does that include routers w/ their WAN facing you (e.g., a downstream router that has access into your network)? Does this include any router that’s been reconfigured as say, an AP, client bridge, or repeater?