請問下如何確定自稱駐港部隊軍人的真偽呢？

There is no need for confirmation.

According to current law and policies, PLA Hong Kong Garrison personnel are not allowed to be active outside their bases. Even allowed, personnel must be in groups. When troubles exist, they must contact their respective commanders, as well as Hong Kong Police. In addition, they are also fluent in Cantonese.

So when a person claims he/she is a PLA Hong Kong Garrison personnel, such chance is extremely slim and most likely the person is fake in term of his/her identity.

軍人代表國家的保護者,起可容許任何人隨意攻擊?真軍人就要敢開槍,只要有對軍人欲以暴行意圖者,軍人就必須先保護自己,驅逐殺死來犯者,才能保護到國家ˋ人民的安全,絕不可貪生怕死,怕與論,不敢開槍的不配為軍人