Systems whose output y(t_0) at timet_0 depends on values of the input other than just x(t_0) have memory.
In other words, a system y(t_0) has memory if its output at time t_0 depends on the input x(t) for t > t_0or t < t_0, i.e. it depends on values of the input other than x(t_0).
Otherwise...