There are times when one might want to tell a lie out of "politeness" but if you suspect someone is a habitual liar, there might be some tell tale signs in the person's handwriting to indicate a propensity to lie more than the average person.  One sign by itself isn't an accurate indication of lying, so look for at least four or five of the following 26 traits and most likely, you'll want to use caution when seeking a truthful answer.  (1) jumbled, impossible-to-read script (2) meandering baseline with light pressure (3) coiled letters (4) changing slant (5) reversed slant or strokes (6) slow, contrived script (7) altered letters (8) jumbled but legible lines (9) unfinished strokes or letters (10) thread-like script (11) arcades in letters or connecting strokes (12) covered strokes (13) flourished letters (14) tapered strokes or words (15) varying sizes of letters (16) improved or corrected strokes in already written words (17) hooks (18) several very different handwritings by the same person (19) capital letters reaching below the baseline (20) end-strokes of "n" or "m" plunging into the lower zone (21) superfluous or heavy dots in the script or at the end of the envelope address (22) very different handwritings in signature and envelope address (23) odd, irregular spacing and pressure  (24) irregular speed of writing (25) "a" and "o" open at the bottom or written with clockwise strokes (26) over sized letter "I".  Source: The ABC's of Handwriting Analysis by Claude Santoy.