How do we know if one is a liar or not then? I mean, how do I know if he or she is not to be trusted if I don't look at Neptune? Is it that because this Neptune influence is part of their personalities, we conclude they are not going to deceit one another on purpose?
I think we need to stop and begin again from the beginning!
Have you any reason to believe that lying and deception are actively going on? Let's call the two A and B. Do you suspect A of actively lying to B or are concerned that A might be lying to B? Or are you just 'interested' to know if A has the capacity to lie and deceive B?
The second is an issue of 'potential', which may never be realised, A might lie but could choose not to. So you might identify the 'potential' but not the fact
The former is much more difficult. It requires both the potential to lie and that that potential is realised. Whilst you cannot ever be sure from simple Astrological evidence, there might well be further information contained in a solar return or other predictive technique which shows that the potential is heightened and therefore is more likely to translate into reality.
Either way the answer is in A's natal chart and its derivatives, rather than in a composite chart, which tells you about the relationship as an entity. In this you might learn about self deception or delusion on the part of the relationship or of the relationship deluding or deceiving others.
Nextly, Neptune is not what you should be looking at, at least not primarily. Neptune, if it has any meaning here, is one of delusion or deception through giving false impressions or false understanding, rather than by deliberate lying.
Lying is Mercury and probably acting in association with Mars, or Venus (or both). To indicate the potential to lie, Mercury needs to be essentially weak - such as peregrine, or in detriment or fall and to be badly positioned tin the chart - say in one of the four places of aversion - 12th, 2nd, 8th, 6th. Lying is not the only way in which an ill-disposed Mercury can manifest, gossiping, sarcasm, constantly putting others down, boasting and being a general busybody are other effects and of course not all manifest equally.
An ill disposed Mercury linked to an ill disposed or weak Mars, may accentuate this, as will a link to an ill disposed Venus, the latter would be especially relevant to affairs, gambling, drinking and other pursuits where A might want to lie about their activities.
Now an analysis here will give some indication of the potential but whether it shows up in action will depend on the strength of the potential and a suitable trigger. Astrologically that will show up in predictive techniques. Assuming that the potential to lie is there at a higher level than normal, we would want to see something in a predictive technique that shows this potential will be triggered and that it will be directed at A's relationship (rather than to friends, or other family members for example).
Clearly judgement requires some care both in the consideration of indicators and the matching of indicators to character or temperament plus observational evidence