how can I tell when the fuel pump is getting weak ?

If you are questioning the pump a fuel pressure tester is the tool you need. They have the adaptors to attach to the test port(s) on your efi fuel system. You can confirm your fuel system pressure at and idle and under load (with help) to help find the issue. Failing pump, filter, regulator. It's not something you would use often ? if you can rent one. If you cannot remember the last time a fuel filter was replaced it might be a good idea to replace that and see if it helps because it should be changed anyway.