May I help you is the best and most polite way to approach someone. That just be your first question. Then you can ask are you lost. If your first question is "are you lost?" It can be taken offensively because you are implying that the person does not belong where they are.

Hey Rio, how are you? Saying ''Can I help you'' to someone is not considered to be an offence at all. Rather, it is a polite way to offer them your assistance. There are other ways to say it, such as: 1) ''Can / Could I help you?'' = in this case, you may sound formal by using ''could'' this way. 2) (Excuse me Sir / Madam, but...) ''(how) May I help you?'' / ''(How) may I be of assistance?'' = ''May'' is considered to be a much more polite word which you can use to make an offer. 3) ''Shall I help you?'' = You should choose this option if you want to sound extremely polite. Using ''shall'' adds an additional degree of politeness or formality to the sentence sometimes lacking with 'could' or 'may'. Hope that helps!!!

Exactly the other way round! "Can/May I help you?" is a polite way to offer your assistance whereas "Are you lost?" might be offensive, in my book that is.