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 handover requires the terminal to first measure various network metrics. A relevant metric is the allocation rate of network physical channels. In the context of OFDMA and based on the physical signal properties, we propose a method to estimate this rate. Detailed numerical examples demonstrate the efficiency of the proposed estimator. Comparisons with a classical energy detector are also returned.