The emerging trend to provide users with ubiquitous seamless wireless access leads to the development of multi-mode terminals able to smartly switch between heterogeneous wireless networks. This switching process known as vertical handoff requires the terminal to first detect the surrounding networks it is compatible with. In the context where these networks are cognitive, this can be challenging since the carrier frequency of their access point may change over the time. One solution to overcome this challenge is to embed network specific signatures in the PHY layer. We here focus on cognitive OFDM systems and advocate to embed signatures onto pilot tones since (i) it makes possible to discriminate systems with the same modulation parameters (ii) it creates easy to intercept signatures implying short detection latency (iii) it avoids adding any side information dedicated to detection that would reduce systems capacity. We propose two complementary signature/detection schemes based on second and third-order statistics, respectively. The first scheme relies on redundancy between pilot symbols and the second is based on the use of maximum-length sequences. Detailed numerical examples demonstrate the efficiency of the two detection criteria in realistic environments.