How to tell?
To identify if your drives are SATA or IDE open up the box and look at the cables connecting your drive to the motherboard. SATA ones are a LOT slimmer, and the connectors a similar size to USB, whereas IDE connectors are about 6cm wide.
As to the speeds, the SATA types are backwards compatible, so if you plug in an SSD you'll still see a performance improvement, just you won't be getting the full benefit of what the drive can do.
If you're asking this question though I'd also ask what OS are you using? To see the full benefit of an SSD you really need to be using an OS less that 3 years old or so.
I put a C300 in my old desktop a year or so ago, which runs on a low-power AMD Athlon 64 laptop CPU @ 2GHz. The box runs fine, and I only remember it's a weedy chip when I rip the occasional DVD...