Does anybody know a way of getting an iPhone firmware version in restore mode, something like f0recast?

I sold a tethered jailbroken iPhone 3GS with 3.1.2, the client brought it to me and know it doesn't turn on with blackra1n, it runs but it shut downs… So I wanted to see if he updated it or not

you can put the phone in recovery mode and then run irecovery -s.
As soon as irecovery connects to the phone, it will display some information.
Look for the iboot number.

Here's a short list for you to compare

# iBoot-385.22 (2.1 and 2.1.1)
# iBoot-385.49 (2.2 and 2.2.1)
# iBoot-596.24 (3.0 and 3.0.1)
# iBoot-636.65 (3.1 and 3.1.1)
# iBoot-636.66 (3.1.1 7C146 and 3.1.2)
# iBoot-636.66.33 (3.1.3)