If you know the port monero-wallet-rpc listens on, you can connect to that port to see whether something is listening. This is what the python RPC layer does.
From utils/python-rpc/console.py in the Monero tree:
# check for open port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
if s.connect_ex((host, port)) != 0: