Visual inspection may work or hopefully someone will chime in with common leak points on this engine.
Smoke a cigar or vape (if you have any dooshy friends) by the engine and see where the smoke gets sucked in. Difficult with the fan blowing though.
Could also cap off the crank case breather, use a 3.5" silicone coupler and clamp a PVC cap with quick disconnect air fitting into in the end, then with a regulator slowly build some pressure in the intake manifold/vac lines, 5psi should work fine. You may hear oil bubbling/air getting past your rings and venting out the dipstick tube, hold your oil fill cap down tight. Then search for the hiss and feel the breeze, this is actually a cheap DIY boost leak test for turbo cars but suppose it could work in this case?