It sounds like you're not totally sure if you hit a car or if this is the right car. Unless you never look at your car, I think it's safe to assume you did in fact hit a car the other day. White cars are very common, so that's not overly helpful. However, blue cars are much more uncommon, and there are many shades of blue. Take a close look at the paint left on the white car. Does it match your paint color? If so, evidence strongly points to you being the culprit, and the right thing to do is 'fess up to it.
Leave a note on their car saying you suspect you hit their car at [date and general time of day] and leave your phone number. Include a description of where the damage is so they know where to find it in case they haven't noticed it yet. Keep it short; they can call you to exchange more info.
If it's rainy outside, put the note in a clear plastic bag so it'll still be legible if it gets wet. Put the note somewhere that it's very visible; it's best if the paper is fairly large and brightly colored. It should be obvious from inside the car in case they don't look at that front before they get in. (This is because you don't want them to find the note by turning on their wipers on the freeway! I've had that happen and it nearly caused me to wreck.)
Once they find the note and inspect the damage, they may decide it's not worth calling you and getting insurance involved. If the do decide to call you, then you can talk to get more details from them and confirm you hit their car. Be honest about what happened, then ask them what time they left that day and where they were parked and see if it matches up. Do not accuse them of lying. It's likely you damaged their car and drove off without checking. You're inconveniencing them, so be polite and cooperate. If they claim they can't remember where their car was or anything like that, then you'll need to suck it up and work with them.
If they give info that makes it impossible that you hit them, then say something like "Oh, I guess it wasn't me after all. Sorry to have bothered you, and thanks for your time!" Don't give them an opportunity to argue about it. I think this is unlikely, though.
If you're worried about your insurance, you can ask to pay for it or fix it yourself. You can say it's minor and that it's easier and cheaper to not get insurance involved. You can sometimes have your rates raised for claims that aren't your fault, so you can try mentioning that if they seem hesitant. If they do agree, make sure you get a written estimate from wherever they take it before they get any work done. If they insist on involving insurance companies then cooperate. It sucks, but ultimately you're at fault here, so it's not right to make their life harder.
In the future, if you even suspect you hit something, take the 60 seconds to get out of your car and inspect things. It will make your life a lot easier! There could have been damage to your car that would have been very bad to find on the road, after all.
