tr.v. Slang sussed, suss·ing, suss·es
To infer or discover; figure out: “I think I’m good at sussing out what’s going on” (Ry Cooder).
To size up; study: “Suss out the designers in whom you are interested” (Lucia van der Post).
c) The OED cites it as a colloquial use of the past participle of “sus” or “suss”
[Abbrev. of SUSPECT v.; cf. prec.
Participles of the verb are usu. formed with a double final consonant in the stem. The form with final double s has now spread to the infinitive. The substantive, however, is still most commonly encountered with a single final consonant (sus).]
a. trans. To suspect (a person) of a crime (cf. SUS n. 1). Also in general use.
b. With obj. clause: to suspect, to imagine or fancy (something) as likely; hence, to feel or surmise.
To work or figure out; to investigate, to discover the truth about (a person or thing). Also with obj. clause and without const.
It may be an archaic term that became obsolete in modern Britain but survived in the Caribbean, (another example is ‘vexed’) and was then revived in 20th Century Britain with the arrival of immigrants from the Caribbean.