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Two time trials, one number. CSS is the single most useful pace you can know — the line between aerobic training and the work that actually improves race speed.
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CSS is the fastest pace you can sustain aerobically — essentially your swimming threshold. Train at CSS and you push that line up. Train slower than CSS and you mostly maintain. Train faster than CSS, you build top-end speed but burn out fast.
The 400/200 method (Ginn, 1993) is the simplest field test: the difference between your 400m and 200m time, divided by two, gives you a remarkably accurate per-100 threshold pace. No lactate analyser needed.
Once a week, do a CSS set: 8-20 × 100 on CSS pace with 10-15 seconds rest. If you can't hit the pace, your CSS dropped — retest. If it feels too easy after a month, retest — your CSS has improved.