A TDS tester will provide a quick, direct and decisive answer whether there is sea water in the cooling system. A pressure tester will provide a direct and decisive answer whether there is a leak in the cooling system. A leak between coolant and sea water will always cause some net loss of coolant. Unless there are recent tests and records, glycol concentration isn't a decisive indicator, and even if so, it's not a direct indicator. If it was me, I'd use the pressure test in any case, by the time all is said & done, because it's predictive. But a TDS tester can be had cheap and may later be handy.
When servicing heat exchangers with removable tube bundles sealed with o-rings, tighten the end caps evenly, a bit each end back and forth. Don't trust the bosses in the end caps to insure alignment.
Experience develops good judgement, bad judgement develops experience