You can't know "for sure", about the best you can do is find the model code sticker on the rear subframe (or on the rear fender under the registration plate), it'll say one of the following.

NSR250RR means its an R (well, it's subframe/rear fender is anyway)

NSR250R2R means it's a red/white or blue/white SE

NSR250R3R means it's a Rothmans SP

NSR250R3S means it's an HRC colours SP

NSR250R2T means it's a dayglow colours SE

NSR250R3T means it's a Repsol SP

R model '28's are quite rare, they were only made in 1994 and 1995, in 1996 they made SE's and did the final SP run, from then onwards they only made SE's.
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