There are a number of ways (including DNA extraction and amplification) to test a urine sample to determine if it is from the purported donor. A blood sample would definitely suffice as a reference for comparison. In some cases where the results are disputed, such tests are done to confirm the identity of the donor of a positive sample. However, it is very unlikely any of these would be done for a standard post-offer pre-employment urine drug screen. It would significantly increase the cost and unless the lab has no backlog or the company pays extra for expedited testing, it could take weeks to get the results. Most drug testing lab facilities do not have the right equipment or qualified staff to do DNA extraction, amplification and sample comparison.
If you try to sneak in someone else's urine, it is much more likely that you will be caught by the standard drug testing security protocols. Most people who try that get caught. It's very difficult to bypass most of the observation and security protocols and the urine sample also has to be in a very narrow temperature range or it will be rejected as invalid.
Depending on how often you smoked marijuana, there is a very good chance that your urine sample would test negative or below the minimum metabolite thresholds for THC after 2 weeks of abstinence. However, if you were a heavy smoker, it could take 30 days or longer.
You really should not take a pre-employment drug screen unless you are absolutely sure you will pass. If you don't pass or if you get caught trying to use another person's urine, you would be wasting their time and yours and probably eliminating any chance that you will ever be offered a job at that company. If there is any chance you will test positive, you would be better off politely declining the job offer (just tell them that you changed your mind) and waiting until you are completely clean to resume your job search.
If you want to find out if you will pass or not, you can buy instant urine THC testing kits at drugstores and online. For example, here is a THC test you can buy online:
These are mostly accurate but not as accurate as a laboratory test.