Checking iPhone 3G bootloader is easy follow steps below.
1. Download BBUpdaterExtreme in this LINK Unzip the file. BBUpdaterExtreme is located inside Update_Baseband_02.28.00 folder.
2. Upload BBUpdaterExtreme in root using DiskAid.
3. Install MobileTerminal in Cydia (if you don't have it yet).
4. Once installed, open MobileTerminal.
5. Type the commands below line per line and hitting return button after each line. Take note of the caps.
6. login/type su
Note: when you type alpine, nothing is really displayed on screen. This is normal. Just continue typing.
8.type: cd /
9.type: chmod 755 BBUpdaterExtreme
10.type: launchctl unload /System/Library/LaunchDaemons/com.apple.CommCenter.plist
11.type: ./BBUpdaterExtreme queryversion
12.You will see the screen below. As you can see, my iPhone 3G has bootloader 05.09
Credits to geohot for everything.
* Applei.ph and Jaytrix for guide