If your external disk interface supports it, a good tool is to use
smartctl. You can get it by installing HomeBrew and then running this command in the Terminal:
brew install smartmontools
smartctl like this:
smartctl -a /dev/diskX
where X should be replaced with the disk number of the SSD in question.
In the output, you want to look for a line like this:
Percentage Used: 1%
This is an indication of the wear on the drive. However, note that it is not an exact science - the drive could wear out before it reaches 100%, and it might also reach values above 100%.
Note: Wearing the drive down doesn't make it slow, as you seem to indicate in your question. When the drive wears out it will fail to be able to read some or all of the data that you have stored on the drive.
Make sure you have a working backup system in place. Just looking at the wear indicator and replacing the drive when the indicator is high won't ensure that you will keep your data.