Maybe providing some information like "how, why, when, and what" would help us share the right answer based on what you are looking for.

Because in general, the most popular I know is https://www.chainalysis.com/. (not sure if that is the legit site).

WalletExplorer could be one good tool too for individual investigation.

This is a very common problem today, which causes a lot of problems for us and those who work in it, due to which they want to get a lot of such things and advice. Regarding this, you will be given a lot of advice here and such tools will be told that will give you and such people a lot of benefit. I have a lot of thoughts about it, but till now I have not found any good tool about it, nor have I found any solved thing about it, but I will give a lot of advice about it, but I will not find any specific reason.

If you go to the groceries, you give $50 and get $23 dollars change, and the clerk gives you two $10 bills and three $1 coins, will you try to check if they were stolen? Or will you simply get them and spend them in another shop?

I want to track funds that were stolen from a MetaMask wallet on the POL blockchain, where USDT was stolen.
Yes, I can pay for some tools.

How it would be defined as "stolen" or not?

Scammers could really be as smart with the tricks of stealing crypto funds but can be traced with diligent analysis comprising of networks which the scammers uses to steal the funds.
Some outsmarting kind may use the decentralized hosts such as non custodial wallets but then, noting that the wallet address would in time interact with the centralized exchanges networks would be a point to hold trace the scammers regarded that if blockchain revealing users IDs can be pseudonymous, they Kyc captions will realistically expose those incline.

And when followed bit by bit will get to the root of identifying the scammers but you must follow through the security agencies because they're the authorities that can only demand for the exchange to hand them over specific users documents after providing evidence that the user is involved or linked to scam.