[BigJimP]

You can't measure the difference very well with a caliper / because the chamber at the back of the barrel is oversized vs the end that is choked.
You really need a barrel "bore" gague - so you can get an accurate measurement inside vs the end that is choked. Some gun clubs have one that they let "members use" ... but if you go out to a club / and somebody is lounging around ...they will probably check it and help you out. It only takes a few minutes.

But those barrels have to be marked somewhere ... Take the barrel off the gun / and you should see some marks on that barrel.

The older brownings use an asterisk (*) or a set of them to determine the choke of the barrel. (one) * is Full, (Two) ** is Modified, and (three) *** is improved cylinder. This marking is usually located on the side of the barrel along the same line as the mfg's name.

More complete answer BROWNING CHOKES AND THEIR CODES (ON REAR LEFT-SIDE OF BARREL)

'*' designates full choke (F).

'*-' designates improved modified choke (IM).

'**' designates modified choke (M).

'**-' designates improved cylinder choke (IC).

'**$' designates skeet (SK).

'***' designates cylinder bore (CYL).