I would not use a stick with a nail on it - puncture electric/cable TV/phone wires ? However, gently push probing with a dull ended metal rod like a piece of rebar or 1/2" metal stock should not hurt but remember the cap is only about 4-5 inches in diameter - may be ABS black plastic, metal, or rubber. Likely the cap is only a few inches down, probably pretty much same place relative to condo as your neighbor's - typically 2-5 feet from building wall, so if in mulch or grvel just clear about a 3 foot circle and you will probably find it.
First place I would look is under or immediately next to front porch or back porch, whichever is closest to street or parking lot - commonly stubbed up there, though not very accessible. Then next to bushes - a lot of people plant bushes to hide the standpipe.
Five ways to locate, in different instances as applicable:
1) track your interior main sewer pipe from kitchen and bathrooms to where it enters the slab or exits the wall - there is usually a cleanout there inside the house. Starting to clean from there, if he keeps track of footage, the plumber will commonly "feel" the outside cleanout as he run down the pipe with a router, especially in cold country where the pipe level drops down several feet as it gets to the cleanout outside. Knowing that distance, finding it outside should be easier. In some cases, particularly with same-level outdoor sewer runs, you can rout all the way from the house to the street from the inside cleanout and don't even need the outside cleanout.
2) camera with a radio transmitter on it, which transmits a signal which you then detect with a handheld receiver walking around - can usually be snaked through a floor drain without S trap, down a cleanout in the sewer pipe in the house, by removing toilet, or by removing trap on sink furthest "downflow" and running in stubout pipe after the trap. This is probably what your neighbor was thinking of. Depending on what types the sewer and drain contractor has (most plumbers don't have them), differnet models can fit in pipes down to 1/2" diameter - certainly can handle undersink stubouts without having to dismount toilet.
3) if metal sewer pipe to street or plastic that had a tracer wire put in with it bonded to metal pipe at the house, some sewer companies will trace and locate the line for you from street to the wall of your house for free even if your responsibility for the line carries all the way to the street, and can locate the cleanout riser pretty exactly if it is cast iron rather than ABS/PVC or clay.
4) get a $10 METAL (not plastic) stethoscope at local pharmacy or box store, and with cold water running pretty much full force in a tub (make sure it will not overflow while you work), listen for water sound. Start in house to detect where pipe leaves house to give you distance from corner or window to listen at outside, then work away from house - may be faint outside house, but should be good and loud when over the cleanout. You can also push a metal rod down in ground a few inches, far enough so it stands by itself, and listen on the top of it with the stethoscope WITHOUT TOUCHING the rod with your hands (will dampen the sound) - works much better than direct on ground. This way you can usually track it all the way to the street. Having someone just tapping on the pipe inside the house works even better, if you have 2 people on the job - of course, not so hard you crack the pipe - a sharp rap with a dowel or screwdriver handle works as good as a hammer. I have even seen a plumber do something really bright when he was looking by himself for a bunch of ABS pipes that had been curied and paved over on a large military project - he taped an electric vibrating engraving tool against the pipe right inside the building and left it on, then followed the buzzing sound with rod and stethoscope to trace the plastic sewer and water pipes up to 30 or 40 feet away.
5) If you have a metal cleanout standpipe, a metal detector will find it - a stud finder that works by detecting metal studs and nails and screws rather than by ultrasound, for instance. Ditto for a regular metal detector if you happen to have one in your hip pocket. Of course, on clay and ABS/PVC pretty useless.