You're mounting this inside a cupboard, so there's some flexibility for exploration.
Start by finding the studs using any way that works for you.
- Studfinder - never had much luck with these personally
- Knocking on the wall - this also routinely fails for me.
- Magnet - get an old Hard drive magnet and slide it over the wall. You'll feel it pull when it goes over a nail/screw. Mark the spot and then search straight up/down for more. Note that the very top and bottom of wall have top/bottom plates, so nails in the edge 40mm are probably NOT studs.
- Shadows - shine a bright light sideways on the wall and look for nail pops.
- Wall plates - if there are any power sockets, ethernet plates, aerial outlets, or light switches on either side of the wall, you can pop the lid off and look inside. I use a thin plastic ruler to slide between lining and any insulation in wall, until it clonks into a stud. Mind out for power.
- Identify existing fasteners into studs - look for big screws for shelf brackets etc.
Once you've identified a stud, drill into it with a thin drill bit - a 2mm or so. If it goes "drill... freespin" then you missed. If wood swarf comes out, then you got something. The difference in feeling is obvious.
If you missed , then grab a length of thin wire. Bend into a curve, feed into the hole, and swing it around. You're feeling for the stud. If you find nothing, stop and reevaluate.
Once you have two guaranteed studs, then measure the gap between them. Your next one should be that distance further down the wall. Depending on the age and location of your house, it could be 18 inches, 24 inches, 600mm centers.
Its also possible that near doors and windows, the studs are doubled up together for added strength. And sometimes a wall's length isn't cleanly divisible by the distance between centers, so sometimes there's a shorter gap.
Inconveniently, cupboards/wardrobes/closets tend to be too small for clean spacing of studs, so, good luck!
Now you have a wall, with studs identified and perhaps a few.... test bores.
You can generally wipe a little putty into the small holes and smooth them off, and then paint in a couple of days. Filling small holes is a lot easier than filling bigger holes.
If your space doesn't have enough studs, then made the spreader board go from wall to wall. This also helps if the rack's mounts don't line up with the studs conveniently.
You're aiming to hang a decent amount of weight too, so I'd suggest getting a spreader board / load board / backing board. A 25mm piece of customwood or stout ply would be ideal. Make sure its wider than your rack, and wide enough to span two studs. You can even run the board right up to ceiling level to catch the top plate - the bottom plate is normally behind skirting board and out of reach unless you remove that too.
Use plenty of decent lag bolts to secure the backing board to the wall via the studs. Use a lot of lag bolts to secure the rack to the backing board too, aim to fill every place in the rack that offers a screw-point.
If that's not enough, you can add stringers along the side walls to add bracing off the
The backing board also lets you add power points where you want them, and to lay out multi-boxes and wires neatly to avoid the ugly Rats-nest.
Personally I got a full-height 42U floormount cabinet used, and it works perfectly. When moving house we just carefully wheeled it onto a truck via the truck's tail lifter. Another option for you is to mount your UPS on the floor, perhaps on a platform to keep it off the concrete. Mine lives in the garage, which helps with ventilation.