on point to point devices, always make sure that the encapsulation matches on both sides, and make sure the clock rate is set on the DCE side (show controllers serial <n> will give you this status).
Check Layer 1 (physical)
Check Layer 2 (data-link)
Also, enabling CDP can also be a good layer 2 check, as it works exclusively at Layer 2 of the OSI model, then you can
Check Layer 3 (network)do 'show cdp neighbors' to see what directly connected devices your router(s) know about.