What's the best test for determining the health of your kidneys, if you've had type 1 diabetes for over 40 years?

Many options. Agree. The easiest screening test is the (urine) microalbumin. You just pee in a cup for this. On your routine blood test, BUN and cr (creatinine) can give you a clue about kidney function. The 24 hour urinary collection for protein and creatinine clearance is the best test to eval for kidney disease (if collected correctly).
Several tests. Serum BUN (blood urea nitrogen) and creatinine are helpful, along with a 24 hour urine for the excretion of microproteins may help to determine whether the diabetes is hurting your kidneys.