No tips on cleaning them, some bosch shops will test them for you but, if you have a smooth idle I would not mess with getting them tested. If the rubber is pliable I would not replace it.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)