Trying things safely.
WHERE TO TEST
A staging site.
A subdomain with a copy of your site, protected with Directory Privacy.
WHY
Installing on live and discovering a conflict is disruptive.
WHAT TO TEST
That it does what you need That it does not break anything Its effect on speed That your contact form and checkout still work
MEASURING SPEED IMPACT
Test before installing, then after.
Some plugins add substantially to page weight.
HOW LONG TO TEST
Long enough to exercise what you actually need.
For a form plugin, submit forms. For a shop plugin, complete a purchase.
WHAT TO DO AFTERWARDS
If it works: install on live, configure the same way, test again.
If it does not: remove it and try an alternative.
IF YOU HAVE NO STAGING
Back up, install at a quiet time, and be ready to remove it.
Not ideal, and better than nothing.