At top level the Olympic rules are testosterone suppression to under 10 nmol/l for at least 1 year.
Many sports follow that, some use 5 nmol/l.
Women's typical level is around 1.5 nmol/l. A woman at 5 nmol/l should be seeking medical attention.
But there's no mechanism in place to enforce that that I'm aware of.
At lower levels, there's no realistic chance of testing - just as you can't necessarily expect drugs tests, and many sports have not been mandating any hormone treatment. Many males competing in college sports have been doing so unmedicated.
And it is almost certainly far from true that lowering testosterone removes any physical advantage. Masculinisation is accumulated - stopping the testosterone doesn't make a man's body suddenly start dissolving.
And males have fundamentally different skeletal structure regardless - even if they were castrated/puberty blocked.
It's no more reasonable to believe that giving drugs to a man will make him perform like a woman than giving drugs to a man will make him perform like a boy, or giving drugs to a horse would make it perform like a man.
A woman is not just a man without testosterone.
Here's one of the key academic papers on the subject recently - a review of previous research
link.springer.com/article/10.1007/s40279-020-01389-3
Hopefully someone else can find one of the easy-to-digest visual summaries of the paper.
Conclusion:
We have shown that under testosterone suppression regimes typically used in clinical settings, and which comfortably exceed the requirements of sports federations for inclusion of transgender women in female sports categories by reducing testosterone levels to well below the upper tolerated limit, evidence for loss of the male performance advantage, established by testosterone at puberty and translating in elite athletes to a 10–50% performance advantage, is lacking. Rather, the data show that strength, lean body mass, muscle size and bone density are only trivially affected. The reductions observed in muscle mass, size, and strength are very small compared to the baseline differences between males and females in these variables, and thus, there are major performance and safety implications in sports where these attributes are competitively significant.