Vostro 3250 is a desktop system. Do you mean Vostro 3520? If so, that model has a backlight toggle on the F5 key. If you have priority set to the standard function keys (e.g. F5 to refresh page), then press and hold Fn button and press F5 key. If you have priority set to the secondary functions of the function keys, then press just F5 key to toggle backlight on/off. There may also be a setting in BIOS/Setup. If there is no icon on the F5 key for a secondary function for backlight (or on another F key, if this is not a Vostro 3520), then you don't have a backlit keyboard. On many laptop models, backlit keyboard is optional, so not all buyers choose it.
My new dell vostro 3250 has no backlit keyboard, and can't find the option in the bios settings, however the seller noted that it has backlit. So is there is any chance to troubleshoot this? I can't find keyboard illumination in the bios, and tried all the fn possibilities without seeing a change.   
Dell vostro 3250, no backlit detected