Brief Answer:
yes, they can detect the difference if they WANT to.

Detailed Answer:
Blood tests are reliable for drug in the system right then. It is positive for about 2 days at most. This is true for alcohol or drugs. It can be quite accurate BUT it does NOT test far back.

There is a cheap urine test that is like a drug store one for pregnancy (and is indeed, available in the drugstore). It is based on making paper specifically sticky for particular drugs. This can only be done for urine and detects hydrocodone (only because one breakdown product of it in the urine is the same breakdown product for heroin). It is NOT reliable to detect OXYCODONE (percocet). It detects for up to about 5 days after last use.

If good tests are done (on urine) then results are different. The mass-spectroscopy test can detect hydrocodone, oxycodone and their breakdown products for up to at most a week after last use (really 5 days). It differentiates between oxycodone and hydrocodone. This test is MUCH more expensive.
That is for blood and urine tests. Hair tests can detect each drug in the system for the time the drug was taken in hair that was growing at the time the drug was present. As such it can go back for months easily. This is the same (expensive) test as with the better urine test.
.
Alcohol is an entirely different matter. There are two groups of tests. One is for alcohol AT THE TIME OF TESTING. It only is positive while alcohol is there. It is like a breathalyzer. It detects alcohol roughly for one hour per drink. It is positive for hours, not for days.
There are alcohol breakdown products that stick to blood and urine. ethylglucuronide is the most commonly used. It sticks around for about 3 to 5 days after last use. It is much longer lasting BUT it is both not entirely reliable and the tests regularly misfire and detect fermenting bacteria that MAKE alcohol in contaminated urine. AND it sorta misfires by identifying drinking that is entirely legal and appropriate (there is no reliable cut off for one small drink a day on the amount of ethylglucuronide made...one could be very far from having a problem and still light up the test). And it misfires sometimes.

So, yes, they can if they want detect particular drugs well. It costs. It goes back about 5 days unless hair is used in which case it is months. Alcohol tests are either breathalyzer or not quite ready for prime-time.

---