I look at the shelves.
If the spot seems a little bare, I'll check my ammo cans to see if anything got left in there after the last trip. If they're empty, I check the "triage center" to see if the brass is waiting to be tumbled. If that's empty, then the brass is waiting to be loaded, or I lost the ammo.
I know, off the top of my head, that I have an approximate amount for certain cartridges. For example, I know that I currently only have about 230 loaded rounds for .44 Mag (2 50-ct boxes 180 gr Sierra HPs, 1 100-ct box Hornady 240 gr SWCs, eight 310 gr WFNs (need to cast more), and some random experimental loads).
But for others, I have no idea. If you ask me how many rounds of 7.62x54R I have loaded with Hornady 174s, I'll give you a blank stare and then an "I have no idea - maybe 2 or 3 boxes?" (I just checked. It's 9 rounds.
)
Generally, the more critically low the supply is, the more likely I am to know exactly what I've got ....because replacing it is on my mind.
20 ga loads? Yep... 2 boxes, and they're both super-wimpy "reduced recoil / reduced report" loads.
.444 Marlin? Umm... 150 rounds? I don't know. There's enough to hunt and punch a few targets for several years, so I'm not worried about it.
.38 Special? Yep... I've been on the same (last) box for 3 (4?) years now. There are 34 rounds in the box, 5 rounds in the J-frame, 11 rounds in speed strips, and 5 rounds in my truck.
.270 Winchester? Uhhhhh... No idea, but it takes up more space on the shelves than any other rifle cartridge.