I thought I heard metal on metal at 77k miles. So I took my car to the mechanic to have him check out my brakes, he said that I still had 50% left on them. So I felt rather stupid. Anyway, the car was purchased used at 71,000 miles, and I have NO maintenance records, in which the Kia dealership (believe it or not) said they would provide for me (I still have to get back to them). Anyway, I don't know when the brakes were last replaced, or if it was serviced at a MB dealership (rather if new brake pad wear sensors were put on).
Well I'm not hearing metal on metal (so I think?) every time I am in a parking structure or am braking close to a wall. No brake lining lights have lit up. Before I make a fool of myself again having my mechanic check it out AGAIN, and have them still be good, are there any other relatively easy way to check the brakes without taking it in? Thanks so much for all the help.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)