how can i tell for certain if the valve seals or valve guides are bad?

how can i tell if i have a burned intake valve or if they're not seating correctly due to carbon crust?

should i just replace the stem seals? can i do it, or is it best left to a pro?

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-lee
'83 300CDT (286k)

former proud owner of:
'85 mercedes euro 300TD
'80 mercedes 300TD
'77 mercedes 280e
'80 mercedes euro 250
'82 mercedes euro 250