I wanted to know if a fast is broken if I taste the aftari (when cooking -Just at the tip of the tongue-) to check if the food is cooked correctly or not?
Imam al-Bukhari has put a statement of ibn 'Abbas (which is not translated on sunnah.com as it is not a hadith he would rely on) saying:
وَقَالَ ابْنُ عَبَّاسٍ لاَ بَأْسَ أَنْ يَتَطَعَّمَ الْقِدْرَ، أَوِ الشَّيْءَ. (Sahih al-Bukhari)
Ibn 'Abbas said: there's no harm in tasting the food from pots or other meals (Source of the translation -see page 96)
This statement (or a similar one) should appear in as-Sunan al-Kubra of imam al-Bayhaqi and was also compiled by ibn abi Shaybah according to al-'Ayni in his 'Omdat al-Qari عمدة القاري (commentary on Sahih al-Bukhari).
There is nothing wrong with a fasting person tasting the food if he needs to, if he does that by testing it with the tip of his tongue, then he spits it out without swallowing anything, whether the one who is fasting is a man or a woman. (Source islamqa #26837)
Nevertheless scholars consider this as markrooh (frowned upon) with certain differences (This is taken from: Al-Jaziri's Islamic Jusriprudence accoding to the four sunni schools p-763 ff.) :
The Hanafis say it is undesirable to taste something no part of which has dissolved and gone down into one's stomach: this ruling applies equally whether the fast being performed is obligatory or voluntary unless it is done out of necessity. It is permissible, for example, for a woman to taste the food she is cooking in order to test its saltiness if her husband is ill-tempered; the same applies to a chef.
The Malikis: Tasting food is undesirable for someone who is fasting, even if one is preparing it. If someone does taste food, he must spit it out lest any of it reach his esophagus. If any of it does reach his esophagus despite his efforts to prevent it, he must make up the fast later if the fast being performed is obligatory, whereas if one deliberately allows some of the food to go down into his stomach, he must make up the fast later and perform expiation as well if this was during Ramadan.
The Shafi'is: simply say if something reaches the person's stomach out of forgetfulnees or by coercion, or due to ignorance the person may be excused under Islamic law.
The Hanbalis: consider tasting food unnecessarily as undesirable else it is not. If there is no necessity which calls for one to taste food and if part of it reaches one's esophagus, one's fast will be invalidated.