If possible, try and work with the complainant to understand the nature of the complaint. For example try and establish if there is a particular time of day or wind direction that is the problem. Also try and establish whether there is any particular smell that they are concerned about. Keep records of any discussions as if the complaint goes to court, the proof that you have tried to work with the complainant should be of benefit.
If you feel that there may be some justification to the complaint, a screening Air Dispersion Model can be run to see if your activities are causing a breach of the local air quality standards. A screening model is a low cost way of quickly assessing whether there is likely to be a problem. If this shows that there is a problem, a full ADM may be required to verify the compliant.