you would be getting fg (final gravity readings) at this point. So if you don't already have starting gravity (SG) or original gravity (OG) readings (which are diff. names for the same thing), it really won't tell you much (like what alcohol level the beer has). And yes you should be able to do it, but you must make sure that your sample is COMPLETELY FLAT. Any carbonation in the sample will throw it off.
O, guid ale comes, and guid ale goes,
Guid ale gars me sell my hose,
Sell my hose, and pawn my shoon -
Guid ale keeps my heart aboon! -- Robert Burns