When walking alongside eroding cliffs in more built up / touristy areas, there's usually big warning signs or closed paths if the cliff is unsafe to walk on. Sometimes it's obvious why, if the path is very close to the sea or has disappeared altogether.
If walking in a more rural place though, it's likely that the cliff isn't inspected regularly and therefore such signs don't necessarily exist. What should I look out for, if anything aside from the obvious (big cracks, paths perilously close to the edge) to determine if such a path is relatively safe to walk on?