Rule #1 in detecting a liar:
"Don't listen to what they say, watch what they do"
My Scorpio and Pluto placements have a sort of antenna of detachment then insight.
Rule #2 in detecting a liar:
"If everything they say they've done worked out perfectly, they are probably lying"
eegads, failure is a part of life, when I hear of a trapeze artist who closed a million dollar sale whilst 1000 feet up on a highwire..they need to stop lying.
Rule #3 "If it seems to good to be true, it still can be good, but they are covering stuff up"
People will directly lie to you to increase your perceived value of them, rare is the person who gives you both the up and downside of whatever it is. Not a lie per se, more a concealment of negative sanctions.
Rule #4 "Desperate people will say anything, if they parrot back to you what you've told them, they are trying to gain an advantage"
Of all things in modern culture, this is the most pernicious, it restrains honesty and genuine rapport.
Rule #5 "The more people talk of their honesty, honorability, and self sacrifice, the faster you should hide the silverware"
Noble traits in people are not unheard of, in fact they make mankind worth bothering with, however when people talk of them all the time chances are good they are trying to gain leverage via "they are soo good, I must be bad" effect.
To all of these though, the key is to take your time in a hurry, look for inconsistencies that are outside of the norm.
I finally ran into that pos Aquarius who owed me a sizable sum of $$$, my human instinct was to...uhm...tell him to stand and deliver.
Instead, I merely cataloged his lies, his lack of performance, and his past, and gave him a gentle warning that it was in his best physical interest to stay away from me from now on.
Thus I write these 5 general rules based on recent experience.