If you put on a disguise, no one will recognize you.
Literal
Disguise do-[if-ば], recognize-not [assertion-よ].
変装すれば is the ば-conditional of 変装する — 'if you disguise yourself.' 分かりません here means 'won't be recognized/found out,' using 分かる in its 'to be identified' sense rather than the more common 'to understand.' Both subjects are omitted.