Easy: you won't need to! It will always be "Je leur ressemble" since "Je la ressemble" is not grammatical :)
If you want to say "I look like her", then you will say "Je lui ressemble". Lui = to him/to her/to it. Ressembler needs an indirect object: see tusharbajaj's comment below and the answers to it.
For a more serious answer to your question though, let's take an example with another verb that starts with an r and that accepts both direct and indirect objects:
Je la rends : /ʒəlaʁɑ̃/ (I give it back (to someone unspecified))
Je leur rends : /ʒələʁ:ɑ̃/ (I give (something unspecified) back to them)
So, what you need to hear to tell them apart is the difference between /a/ and /ə/ (these are basically the a and the e in the English word father, except the a is shorter), and the longer r sound in "Je leur rends". Google Translate's audio is quite good if you want to practice.
spectacular help...father has but the a and e sounds for french is an example to remember.
No: les is accusative (direct object), while leur is dative (indirect object). Since ressembler is used with à (as in "Tu ressembles à ton père", you look like your father), it takes an indirect object, so you do have to use leur. Basically, leur means "to them".
That's a good explanation Fortune. I discovered in a previous question that ressembler requires à + an indirect object, it didn't occur to me, until I read your answer that it will always end up with an indirect pronoun when substituting for that indirect object (of course I guess).
Would rather be "Je ressemble à eux" (accent on the 'à') but it sounds like the speech of a baby learning the language: not really wrong, really understandable but never used.
This is what confuses me, in an often quoted article explaining direct object (https://forum.duolingo.com/comment/1124712), the OP uses the example of "I love them" - "je les aime" as an example of a direct object.
Is this not the exact same thing? You are putting the action of the verb, to resemble, directly on the object. Why would you then use the indirect object form?
This brief article sums it up pretty well: http://french.about.com/od/grammar/a/objectpronouns_3.htm
"I gave the kitten to my uncle". Kitten receives the action of the verb directly. To my uncle is the indirect object.
Direct object is the receiver of the action of the verb. Indirect object is a "third-party" involved in the action.
I write a letter (Direct object) to my mother (Indirect object).