In addition to the above great answers from David Siegel and user6726, on top of the F.T.C.'s Used Car Rule which, specifically for California, a 1969 case of a Hungarian descent plaintiff Laczko that is since a corner-stone for statute-based "nonconsensual duty-breach" tort liabilities in the state (and possibly beyond):
This alleged misconduct violates Vehicle Code section 28051, effective November 8, 1967, which then read: "It is unlawful for any person to disconnect, turn back, or reset the odometer of any motor vehicle with the intent to reduce the number of miles indicated on the odometer gauge."  A tort in essence is the breach of a nonconsensual duty owed another. Violation of a statutory duty to another may therefore be a tort and violation of a statute embodying a public policy is generally actionable even though no specific civil remedy is provided in the statute itself. Any injured member of the public for whose benefit the statute was enacted may bring the action. (See Hudson v. Craft, 33 Cal.2d 654, 660 [ 204 P.2d 1, 7 A.L.R.2d 696]; Biakanja v. Irving, 49 Cal.2d 647, 651 [ 320 P.2d 16, 65 A.L.R.2d 1358]; Wetherton v. Growers Farm Labor Assn., 275 Cal.App.2d 168, 174 [ 79 Cal.Rptr. 543]; McIvor v. Mercer-Fraser Co., 76 Cal.App.2d 247, 253-254 [ 172 P.2d 758].) [1b] We hold that respondent's breach of its statutory duty to appellant [not tamper with the odometer gauge of the appellant's vehicle] constituted an actionable tort." (Laczko v. Jules Meyers, Inc., (1969) 276 Cal.App.2d 293, 80 Cal. Rptr. 798 (Cal. Ct. App. 1969))
The case was later summarized in Potter v. Firestone Tire & Rubber Co. (1993) 6 Cal.4th 965, 1018) where the court stated that:
"[C]ourts may use a legislative prohibition as the basis for recognizing a new intentional tort, or for expanding the scope of an existing intentional tort to cover the prohibited conduct. (See Smith v. Superior Court (1984) 151 Cal. App. 3d 491, 497-500 [262 Cal. Rptr. 754] [intentional spoliation of evidence]; Middlesex Ins. Co. v. Mann (1981) 124 Cal. App. 3d 558, 570 [177 Cal. Rptr. 495] [violation of fiduciary duty]; Czap v. Credit Bureau of Santa Clara Valley (1970) 7 Cal. App. 3d 1, 6 [86 Cal. Rptr. 417] [unfair collection practice]; Laczko v. Jules Meyers, Inc. (1969) 276 Cal. App. 2d 293, 295 [80 Cal. Rptr. 798] [tampering with vehicle odometer[is a tort])
For someone who ends up in a situation as in this hypothetical question, it would be great to get expert opinions, and probably best through an attorney. If for any reason, financially neither is attainable, as an alternative, it might be helpful to maybe get car dealers to tell what they would think the approximate odometer reading of the vehicle is. If it is way off, they will be able to tell. If a pro tampered with the odometer, they probably made sure that the amount is within the realm of a reasonable amount so there is a chance that, with substantial certainty, only actual expert opinion and the use of scientific methodologies could yield affirmative proof. The burden in California is by "clear and convincing" evidence generally for civil matters. It might be possible that 70,000 miles and a third of what the car ran will be ascertainable, but probably best would be to get an expert opinion.
These cases are typically handled by "auto fraud" law firms; in many cases, "lemon lawyers" will have overlaps with this field. One better than average firm in the field would probably be Rosner, Barry and Babbitt LLP although it is likely one will simply get stuck with the less savy in-takers or non-partner attorneys. These matters typically are such that are done on a contingency; the draw back is, you sign over full control of the case, except for accepting a settlement. However, there are caveats, and lawyers typically don't like that contratual right to be used the way that doesn't allign with their interest. Typically, if one doesn't want to settle, they will want the customer to pay them out for their work. So effectively one doesn't really have a right to decide whether they want to settle or not. If the only objective is to get ride of such a car, and one looks at, say, punitive damages or civil penalties as a plus, and don't seek to pursue, this framework should do the job.