In order to check your IPhone Bootrom version, you must start your iPhone in DFU mode
How to put your iPhone in DFU mode :
1. Connect your iPhone to your computer.
2. Turn iPhone off.
3. Hold power and home together for 10 seconds (exactly).
4. Release power but keep holding home until the computers beeps (observed on a PC) as a USB device is recognized.
5. A few seconds later iTunes should detect your iPhone.
6. If the Restore Logo is present on the screen, you are in Recovery Mode, not DFU.
7. Go to Device Manager, USB Controller -> Apple Mobile Device USB Driver and right click Properties and goes to Details Tab. If it is at DFU mode, Device Description will say Apple Recovery (DFU) USB Driver
8. Now, goes to Device Instance Path USB\VID_05AC&PID_1227\CPID:8920_CPRV:15_CPFM:03_SCEP:03_BDID:00_ECID:000003E92601A3CF_SRTG:[IBOOT-359.3.2]
9. IBOOT-359.3.2 is the bootrom version