This text was pretty complete:
What is a wolf tone?

Wolf tones occur when strong sympathetic vibrations from the
instrument itself interfere with string vibration. The sensation may manifest itself in pulsation, throbbing, roughness, jump in
frequency, or difficulty in drawing the tone from the instrument.

To a greater or lesser degree, wolf tones are present on all instruments, even the finest Stradivari, caused by excess tension, or an anomaly in design or graduation. Typically, wolf tones can be heard (and felt) when playing B or B flat on the violin, B flat or C on the viola, and E to F sharp on the cello (especially in fourth position on the G-string.)

Most good players learn to compensate for the wolf tone; proper Vibrato can often make the wolf disappear; cellists often simply squeeze the lower bout with a knee when playing in areas where the wolf lurks.

What can I do if my instrument has a bad wolf tone?

Adjusting or refitting the soundpost or bridge, installing a thicker soundpost, or fitting an internal wolf resonator can help tame the wolf, but, before taking drastic steps, try the following options:

First, make sure that the instrument has no open seams or areas that have come unglued. A loose soundpost can often be the culprit, and may be caused by a loose bottom seam on the treble side, or even too much humidity, which causes the instrument to swell.

If the instrument is sound, try:

1. Changing the offending string to a thinner gauge string.

2. Using a Si-Hon style mute, which dampens the area around the tailpiece, or twisting a Tourte-style so it wedges between the strings.

3. Fitting a wolf-tone eliminator on the string behind the bridge. Moving the eliminator closer or further from the bridge can alter the pitch, and by placing it on a quarter tone or less vital note, reduce the frequency of the wolf to some degree. Once the optimum location is identified, the eliminator can then be locked in position by tightening the adjustment screw.

4. Altering the sympathetic vibration of the strings. One way is to fractionally lengthen the tailpiece loop (which will slightly shorten the overall string length).

5. Using a heavier tailpiece. Often, switching from a synthetic tailpiece to an ebony or metal tailpiece will noticeably reduce a wolf tone.

Wolf tones are dependent on an exact pitch which corresponds to the frequency in which the plates are tuned at. When playing fast the tone doesn't usually resonate long enough to notice a wolf and when sitting on the note, many vibrate so it's not always easy to notice a wolf without actually testing long notes, chromatically and perhaps even at smaller intervals played non-vibrato. I'm one of those guys that could miss it