I usually figure a bar is unsafe if there's a fight in progress, I
see bulletholes in the windows, or when there's a mixture of HD's and
souped up rice-burners parked out front.

The USCG, however, has different standards.

I think the formula comparing the wave height to the LOA divided by 10
and then adding the freeboard is pretty useful.
If I follow the logic correctly, the formula predicts that a 30-foot
boat with a 4-foot freeboard should be OK in up to about 7-foot waves.

From a USCG item on bar safety:

**
Many boating accidents and fatalities are attributed, at least in
part, to hazardous coastal bar conditions. A federal law was enacted
in the early 1970s establishing the ocean entrances to northwest
rivers and bays as Regulated Boating Areas under the National Boating
Safety Act. This law gives the Coast Guard authority to suspend or
terminate the voyage of recreational vessels and certain passenger
vessels that attempt to operate in these areas when hazardous or
unsafe conditions exist. Failure to comply can result in a $1000 civil
penalty.

When imposing restrictions, the Coast Guard first evaluates the
conditions at the bar. Restrictions are then based on the type and
size of vessel and the following criteria:

Wave height four feet or greater, or,
Wave height is greater than the length of the boat divided by 10 plus
the freeboard, or,
The surface current is four knots or greater.
These conditions are the minimum criteria for what is considered
unsafe.

**