My dad bought a 1998 Malibu that has power locks on it. My question is whether or not they came with keyless entry as stock. It seems to be a base model with the 3.1L engine. It only came with one key and that's all it had. Any help is appreciated.

if it's a base model and not an LS even though it came with power door locks it probably didn't come with keyless entry. I think only LS had that from the factory. You could install an after market keyless entry, some dealers did that to the cars when new. I think the car came with at least two keys, and probably someone lost the other.

Is there any definite way to know for sure if it has the option or not?

Yeah, there is a RPO sheet sticker somewhere on the vehicle I forget where, I think near the spare tire. But you have to have the info to decipher the codes. Also the vehicle's build sheet can be looked up for it's VIN. I don't have this info but a Chevy dealer would. you could try googling "GMvis" and see if you can find a forum where someone has the info. If it's a abse model I don't think keyless was a factory option. If the dealer installed an aftermarket one, I don't know if they'red be a record of it with the GMvis. I think the keyless control box is normally located on the rear deck.