Green mangoes that are very tart and only slightly sweet (มะม่วงดิบ/มะม่วงเปรี้ยว) are perfect for Thai cooking.
But aren’t all green mangoes tart? Not necessarily. In Thailand, we have another category of green mangoes that aren’t so tart and only moderately sweet (มะม่วงมัน); these are great eating mangoes but not suitable to use in cooking. These non-tart mangoes aren’t available — at least not widely — outside of Southeast Asia as far as I know, so for most people, differentiating non-tart green mangoes from tart ones isn’t an issue due to a lack of choices. This, however, seems to be a problem among foreigners living in Thailand who are overwhelmed by the vast variety of mangoes.)
When choosing green mangoes, it is very important that you choose ones that do not yield at allwhen squeezed. Just because a mango looks green on the outside doesn’t mean it’s an ideal candidate for a dish that requires green mangoes. The mango needs to be very, very firm. And you won’t know that unless you pick it up and press your finger on it.
Yes, be a fruit fondler.
Most of the time, recipes that call for green mangoes instruct you to grate them. A mandoline comes in handy here as you need long, thin strands instead of short, stubby ones produced by a regular cheese grater.