If it says 5.3+ then the developer has marked the add-on as compatible with 5.5.1.

If you have a version range (like 5.3-5.4.2.2) then it won't be compatible with 5.5.1.

A lot of basic addons simply work, though the edit edit interface can be ugly. Those that have dashboard interafaces can give problems. Those that have extensive styling can need some css adjustment. Those that use extensive javascript may break the 5.5 toolbar. Those that work with scrapbooks, beware.

The market place guidelines at the bottom of each addon show whether something has checked to work with 5.5.

And if you are still in doubt, contact the developer.