Generally, if you have a chain that you're confident was cut correctly, just count links and go for it. But it's so simple to check if you're not sure. What I would do - before removing the old chain is to take 30 seconds to do two things:

Shift to the small/small combination. The RD derailleur will be angled backwards quite a bit. Make sure that the chain coming out of the derailleur that is returning forward to the crank doesn't interfere/touch the derailleur pulleys. If it interferes, the chain is too long and you should make the new chain a link or two shorter. Sure, you probably wont be using this combo, but it will indeed give you the maximum length that will work. Called the "small small" method.

Carefully shift to the large-large combination. The rear derailleur will be angled forward. Do the last rear shift carefully, watching to make sure that the chain makes at least a little S curve from the cassette sprocket and through the two RD pulleys. It shouldn't make a straight line. You're trying to see that the chain isn't trying to pull the RD pulley cage forward beyond it's ability. (if that makes sense). If you think that's happening (no curve to the chain, putting stress on the RD), the chain is too short and the new chain should be a link or two longer. This is a version of the "big big" test and tells you the minimum length of the chain that is safe to use.

When in doubt, it's better to make the chain a link too long than a link too short. You'll get noise in the small-small combination but (1) you probably won't be using it anyway and (2) it's not going to cause catastrophic damage like a too-short chain could do if you use it with the big-big combo.

Just do those two tests, they take a second, and you'll know if it's good enough to just count links or if you should add or subtract.