- Allah (ﷻ) placed signs all throughout creation that Islam is the true religion. Qur'an 45:3:
إن في السماوات والأرض لآيات للمؤمنين (Indeed, within the heavens and earth are signs for the believers)
So even before asking Allah (ﷻ) for a sign, already the signs are all around you. Truth be told, the natural order of the world itself debunks many human theories. That is just 1 sign of countless.
Nevertheless, despite the signs being all around us, is it still okay to ask Allah (ﷻ) for a sign? Yes. Even Prophet Abraham عليه السلام asked Allah (ﷻ) for a sign to be more firm in faith.
وإذ قال إبراهيم رب أرني كيف تحيي الموتى قال أولم تؤمن قال بلى ولكن ليطمئن قلبي قال فخذ أربعة من الطير فصرهن إليك ثم اجعل على كل جبل منهن جزءا ثم ادعهن يأتينك سعيا واعلم أن الله عزيز حكيم And (remember) when Ibrahim (Abraham) said, "My Lord! Show me how You give life to the dead." He (Allah) said: "Do you not believe?" He [Ibrahim (Abraham)] said: "Yes (I believe), but to be stronger in Faith.." -Qur'an 2:260.
Both Muslims and non-Muslims can ask ALLAH (ﷻ) for a sign; indeed, some non-Muslims do and are guided to Islam thereby.. because of their dua, their seeking guidance and good within them.
- What's NOT permissible to do is to go summoning Jinn for a sign. Because from among the Jinn, usually the devils descend and what they descend with is lies and deceit. So it being haram, that would be a major sin on you to summon them and then you would be deceived and misguided.
As for Angels, we do not see them in their true forms. So if you're hoping for that as a sign, it is not going to happen. -- Your only option is asking Allah (ﷻ) for a sign. And there are various ways He may send one.
- Keep in mind, even if these signs benefit you... becoming a "convincer of non-Muslims to accept Islam" is going to take more than just showing them signs. Some people see signs and yet they do not accept them!
سأصرف عن آياتي الذين يتكبرون في الأرض بغير الحق وإن يروا كل آية لا يؤمنوا بها وإن يروا سبيل الرشد لا يتخذوه سبيلا وإن يروا سبيل الغي يتخذوه سبيلا ذلك بأنهم كذبوا بآياتنا وكانوا عنها غافلين (I will turn away from My signs those who are arrogant upon the earth without right; and if they should see every sign, they will not believe in it. And if they see the way of consciousness, they will not adopt it as a way; but if they see the way of error, they will adopt it as a way. That is because they have denied Our signs and they were heedless of them) - Qur'an 7:146).
You will have to be well-equipped in both Islamic knowledge as well as the views/arguments that they have! -- I have debated many non-Muslims for the past few years (mostly atheists and Christians). Some of them, despite conceding that I was making valid points, did not accept the conclusion. Lesson: Guidance does not come unless they want it.
I hope this answers your question. In summary, yes... you can ask Allah (ﷻ) for a sign.