First off, good on you for having such a healthy attitude to constructive criticism and feedback. Many people don't realise they can't be above average at everything and so take negative feedback as a personal insult or allow it to knock their confidence rather than spur them on.
My view in performance reviews has always been "tell me the bad stuff". Yes a little praise is nice (and the salary it brings is nicer), but that's the stuff I already have a handle on so let's not dwell too long on it. If I don't know where I need to improve, how can I concentrate on improving that aspect of my performance.
To get to your question: "Ask them" is the easy answer.
However, if they are personal friends or even just decent humans, they are likely to sugar coat their thoughts to (a) protect your feelings and (b) protect their own brand/career/reputation. This is likely to happen no matter how much you insist you want them to be completely honest.
My suggestions would be to
- Ask them for areas where they think you can improve or strengthen your skills. This will allow them to offer general criticism without calling out explicit incidents or expressing exactly how negative they are being.
- Speak to them individually and explain your are looking for a full and frank, no holds barred assessment of your strengths and weaknesses. Explain that you want them to be brutally honest but you don't want to put them in an awkward position of criticising you in person. Then ask if they would complete an anonymous survey that N other people have also agreed to complete. Some people will politely refuse and some will still sugar coat their feedback but you are more likely to get the real honest answers. (Assuming that's what you're after?)
(Obviously if you go the survey route, you secretly send each participant a different survey so you know exactly who said what and can begin planning your revenge. This is a joke - don't do that. It would be deceitful and, to be honest, you're probably better not knowing who said what).