In Vivado versions prior to 2014.1:
To check if the I/O registers are placed into IOBs, run "report_io" or refer to the "IO Report" (*_io_placed.rpt) after place_design and check the "IOB Sequential Element" column.
You can use the following command to extract the information from that file:
In Vivado versions from 2014.1 to 2015.2:
There is an ultrafast app inside the Tcl Store which includes a command called report_io_reg.
In the output table, there are ILOGIC and OLOGIC columns which indicate the IOB register.
To run this command:
In Vivado versions starting from 2015.3:
Starting from 2015.3, the IOB registers information is put in the report_datasheet report.
The "(IOB)" mark in the "IO Reg Type" column means that the cell is placed in an IOB.
This report can be generated by running "report_datasheet -name" in the Tcl console or by generating the report_timing_summary report with the "Report Datasheet" option checked.
Both the GUI report and the text report contain the I/O Reg Type information.