PED/Steroids via his Canadian doctor who was busted entering the US with those drugs.
As you noted, simple observation was all one needed. interestingly, the PGA implemented PED/drug testing AFTER Tiger won US open at Torrey Pines - his 13th major “win”.
Of all the doctors Tiger could have consulted, he chose a Canadian crook?
Tiger was a cheater - in golf and in his personal life - and a despicable human being. His golf “records” will forever be tainted.
There are many who believe he walks on water...including many freepers.

Toddler Tiger on the Mike Douglas Show with Bob Hope, where PED equals Pediatric
It is kind of funny how Mark McGuire and Jose Conseco were vilified, but Tiger, deified.
In his peak years, he was spitting on the green, next to the hole, where others would have the ball roll through. Pretty disgusting. I bet he didn’t do that before steroids.