The best thing is too take the core
out of the heat-exchanger and look down the tubes. If you see lots of good size bumps inside or around the tubes then it's due for a cleaning.
You can either take it to a radiator shop or buy some radiator flush and soak it in a bucket with hot water and the mix. If the bumps remain after a do-it-yourself then take it to a radi shop. They'll need to manually clean the tubes with a scraping rod and they should test it afterwords.
Don't forget to buy new gaskets for the end caps and use anti-sieze on the bolts when reinstalling. And don't tighten the bolts too tight (aluminum heat-exchangers)