Both classes have فَعَلَ as the perfect, so their imperfects distinguish them:
نَصَرَ ← يَنْصُرُ — ḍammah on the second radical in the imperfect: yafʿulu.
ضَرَبَ ← يَضْرِبُ — kasrah on the second radical: yafʿilu.
The imperatives اُنْصُرْ and اِضْرِبْ consequently differ in their initial hamzah vowel.
If you know only that a verb has perfect faʿala, you cannot always determine the class without its attested imperfect.
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Why naṣara and ḍaraba differ
Verb Classes and the Six Unaugmented Triliteral Patterns
Both classes have فَعَلَ as the perfect, so their imperfects distinguish them:
نَصَرَ ← يَنْصُرُ — ḍammah on the second radical in the imperfect: yafʿulu.
ضَرَبَ ← يَضْرِبُ — kasrah on the second radical: yafʿilu.
The imperatives اُنْصُرْ and اِضْرِبْ consequently differ in their initial hamzah vowel.
If you know only that a verb has perfect faʿala, you cannot always determine the class without its attested imperfect.