<p>How can you tell whether the professor will write you a LOR that would help you significantly in the admission process, even if you're close with him/her personally? And what do professors look for when writing their recommendations?</p>
<p>Hi! 
I don’t understand why you ask “how to tell if he/she is going to write good letter” if you know him/her personally/for a long time. You should know this, right? It’s like asking:</p>
<p>“Hi Dr. Derp,
I did research with you for 4 years during my undergrad and we published 3 nature papers together. I’m going to apply for grad school. Can you write good letter of rec for me? Thanks”
obviously he will write you good letter.
or
“Hi Dr. Derp,
I did research with you for 4 years during my undergrad although I only work 1 hour per week to clean dishes and water your Arabidopsis. I’m going to apply for grad school. Can you write good letter of rec for me? Thanks”
obviously he will not write very good letter</p>
<p>If you don’t know him/her well, however, ask him/her **not **only if he/she will write letter of rec, but specifically whether he/she will write GOOD letter. They will understand what this means. This means if he/she write so-so/bad letter they should refuse to write. Or at least that’s how it’s supposed to be.</p>
<p>Prof write based on what you’ve done that might be good/bad; it’s like rating an item after you buy them on ebay. If you got A in class, come to office hours asking good q, then they will write about this. If you work 12/7 in lab for 3 years trying to develop best method in purifying arsenic dna, they will write that you have good working ethics.</p>