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.
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).
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