I think EXISTS would be the clearest indication that you just want to know if a matching row exists, not retrieve data from that row. For example:

SELECT

p.ProductID, CASE WHEN EXISTS(SELECT 1 FROM @ProductRelation pr

WHERE p.ProductID IN (pr.FirstProductID, pr.SecondProductID))

THEN 'Linked' ELSE 'Not Linked' END

FROM @product p

ORDER BY

p.ProductID

I suspect this also might perform better on large table(s), but I prefer it just as much for the clarity of showing what's actually being looked for.

Edit: Reformatted the EXISTS() to fit better in the code window.