You could use the neighbors function, which gives a list of connected nodes for a given node. You could combine this with the ismember function to get a logical result it your both nodes are connected or not.
G = graph(bucky);
N = neighbors(G,10) % show neighbors of node 10
check if 3 is a neighbor:
ismember(3, neighbors(G,10))
check if 12 is a neighbor:
ismember(12, neighbors(G,10))
