1. Harassment (whether it's sexual, racial or whatever) is about how someone feels – whether a hostile, humiliating or offensive work environment has been created, whether someone's dignity has been violated.
2. You don't have to intend to upset or offend someone to harass them. The vast majority, in fact almost all, of the people we've worked with over the years who  have been accused of harassment have said 'I didn't mean to ….' or 'that wasn't at all my intention'. Our job has been to explain that what they intended, or what they foresaw (if in fact they actually thought about the potential impact of their actions) is actually not that relevant.  It's how the other person feels after what they have done that is the main issue.
3. If the alleged harassment doesn't involve email, there's never going to be any 'smoking gun' type evidence.  What you will generally have is a number of differing versions of what actually happened.  Most of this stuff does not happen in front of witnesses and when it does, witnesses frequently give statements that strongly reflect their perception of the parties involved. 
4. A workplace harassment investigator's job is to decide who they believe, who is the most credible witness?  If they fail to do that, they are letting down the organisation and their decision is going to be the sort that helps no-one who is interested in challenging the status quo or creating workplaces in which people can thrive: 'party A says this, party B says this and there is no compelling evidence that requires me to decide one way or the other.' 
5. Harassment on the grounds of sex, race, etc. is a civil wrong and the burden of proof to use in a workplace investigation is the civil burden – is it more likely that version A happened than version B? The specific details of the Rennard situation meant that the independent QC employed by the Lib Dems used the criminal standard of proof: is it beyond reasonable doubt that A happened?  For the reasons stated above, this is unlikely to provide a result.  Had the civil burden have been applied, the QC’s own comments suggest that the outcome would have been quite different. 
6. There are tools that can help investigators in deciding who to believe.  In the Rennard case, the fact that there are apparently four people giving 'broadly credible' evidence of wrongdoing is probably of great significance. In less clear cases where two people's evidence simply conflicts, an investigator should look first at whether the parties have been consistent in their stories and whether what they are saying is consistent with any documentary evidence.  If that doesn't tell you who to believe, a really useful tool is for the investigator to consider: 'if person A is telling the truth, how likely is it that person B would have acted as s/he did?' And vice versa.  This will frequently be a very fruitful line of enquiry. 
7. You always need to think very carefully about how to convey the result of an investigation because workplace splits or schisms often develop.  People in a workplace will have strong views about the parties already, they will know who they prefer and who they are going to side with.  What is often critical is to make clear what the investigation has actually found.  Think about publishing what the investigation has actually concluded.  It is probably not that person A is a sex pest, a racist or whatever; what has been concluded is probably that on one, two or however many occasions, person A is believed to have done something that upset or offended person B and on that basis person A has been disciplined.

And one final thought for the Lib Dems to consider. In the post investigation mess that organisations typically find themselves, suing is not the answer. Trying to find out who did what, who is responsible, who is to blame is not going to be helpful. A forward looking resolution focus is needed – mediation is the only answer.  All parties: Lord Rennard and his supporters, the people who complained about him and their supporters, the leadership – they all have interests and positions. A court can't put Lord Rennard on a particular committee or fashion a carefully worded statement of apology or suggest that the recommendations of the excellent report that Helena Morrissey has conducted into the party should be implemented.  A mediator could do all of those things.