Signs of coning
Coning can be observed visually and may result in the dissolution profiles showing incomplete and variable release.

Coning can occur with immediate release disintegrating dosage forms with high levels of dense excipients, such as calcium diphosphate, or drug products containing a drug with a high density.

In marketed products, it is a relatively rare issue because the issue would have been eliminated in development of a QC method. Coning is also more likely in development to occur in a USP vessel if the dissolution medium volume is too low.

Potential solutions to coning
In some cases, increasing the rotation speed (to 100 rpm) increases turbulence/mixing which breaks up the mound exposing the drug sufficiently to the medium. For biorelevant testing this can a good option providing one is not losing discriminatory power .
An alternative approach is to use a PEAK (APEX) vessel.  This is covered in more detailed here.