Is it possible to detect high impedance at FPGA input?
From the data sheet, https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/hb/cyclone-iv/cyiv-53001.pdf, Table 1-12, Note 1: 


  All I/O pins have an option for a weak pullup resistor


So you just need to learn your tools well enough to enable it.