how can you tell the difference between a SL1 and a SL2?

sl1 are usually single camer slammers. and sl2 r twin cam. sl2 might have better seats but they do have power options and some have sunroofs.

There are more. SL1s didn't get the light bar across the trunk, nor painted door handles. The SL1 doesn't have a rear anti-roll bar. The tach numbers on an sl1 only goes to 6K or something like that. Early cars had unpainted bumpers of course. Remember power windows and locks, ABS with rear discs and alloy wheels could be added to the lowly SL1. Most times that best way to tell is just a quick look at the engine, but most times you can tell from the others things listed.

Body trim,SL2 puts out more hp.

With a first generation SL2 vs SL1, the trunk lid has that reflector look extended from the brake lights, and you can see "Twin Cam" written on the orange portion. An SL1 doesn't have the letters or the reflector on the trunk lid. Second gen is a bit harder, I only know you can pop the hood and see a different cylinder head for the second cam. The twin cam is slightly more powerful, and has a sportier transmission (At least with the manuals, not sure about automatics), and has slightly lower fuel economy, (If the car is in good condition, a typical SL2 on the highway is 35-38MPG, verses the SL1's 38-42). The tachometer will be higher on an SL2(~6000 redline) than an SL1(~5000 redline) Also, only the SL2 has a rear sway bar, and if I remember correctly, had slightly larger standard wheels, although the 15" was an option for the SL1. I'm pretty sure you can check the VIN code to prove beyond a doubt by reading the engine letter, but I usually just stare at the motor for a while.

Like other people said, looking at the engine will tell you 100%. If you just have pictures, painted bumpers usually, but not always, indicates an sl2

Twin cam's redline is 6500, single at 5500 rpm. TC speedometer 130mph but SC is 110. (You needn't open the hood).

This is what a SL2 engine looks