How can we be sure that these things are not monitoring your clipboard, your keyboard when you type totally random words that follow a pattern like 12 to 24 words? Or a screenlogger that takes a screenshot when it detects a seed on the screen and instantly sends it straight to the attacker's server? We know that it is possible to develop this.
If your computer/smartphone is infected with some malware/virus, sometimes there are signs that can point to it, such as increased consumption of RAM or processor, opening of programs that you did not start, redirection to random pages in the browser...
However, the only way to be sure that you really have a problem is the hard way, which means that you create a wallet, send some BTC to it and you are left without everything. However, there is a difference in whether the malware is programmed to automatically steal every small amount that appears in your wallet, or whether the hacker will monitor your balance and empty your wallet only when a larger value is stored in it.
I always thought about this when creating my wallets, I know you will answer that an HW is the best option in this case, yes I know that too.
HWs are definitely a higher level of protection, although we can discuss their security protocols and shortcomings here - but also emphasize that even such devices are not immune to human carelessness and stupidity. The most common way for people to be
"hacked" while using HW is to be fooled by clipboard malware or to fall for the cheap trick of fake support that asks them to enter their seed in something that looks like the original interface.
If someone cannot afford a hardware wallet or make an airgapped wallet from an old computer, then the best way to avoid exposure to online dangers is to stay away from all those things that are risky, which means no porn sites, downloading torrents and clicking on any suspicious links.
Also, forget about inserting USB sticks and portable HDDs that come from external sources (friends, family members) because they can be infected even without their knowledge. Also, turn off Bluetooth when you are not using it, and protect your wireless home network well and do not connect to any public networks.