I have a three month old son and I am STILL trying to find out when he is done eating. He never pulls of the breast on his own.

if your nipple falls out of his mouth he is asleep.

Does the suck swallow slow to almost nothing? Done eating.
Does the suck pressure change from, "OMG he going to remove my nipple" to anything else? Comfort nursing.

If you are comfortable with it, you may consider a paci. If you are not comfortable with the paci- then he is going to use your teat to soothe himself. Pacis simply replace the breast in regards to the self soothing suck.

If you no longer hear him swallowing, he's done.

If he's fallen asleep then pretty likely he's done. If you unlatch him with no major objections, he's done.