Your '95 might be either OBD I or OBD II, if OBD II there will be a diagnostic port along the bottom edge of the dashboard roughly where your left leg rests while driving. You can use the diagnostic scan tool at Autozone for free if you have OBD II, or you can purchase the OBD I tool for about $35. Either way, the scan tool is your friend and you need to use one.
HINT OBD II is required on all vehicles offered for sale in the USA from the 1996 model year. Toyota, the builder of your Geo Prism, elected to make OBD II available in some of its models beginning with 1995 models.