spy

[1]	spy on/upon , follow , shadow , trail , watch , observe , reconnoitre , keep under surveillance , (US) surveil , (Colloq) tail , check out , case
[2]	espy , descry , spot , glimpse , spot , espy , catch sight/a glimpse of , descry , note , notice , see , discern
[3]	stag , snoop , sleuth
[4]	sight

[1]	double agent , undercover agent , foreign agent , secret(-service) agent , intelligence agent , undercover agent , mole , fifth-columnist , CIA man/woman/agent , MI5/MI6 man/woman/agent , informer , informant , (Colloq) mole , (Slang) stool-pigeon , stoolie , fink , ratfink