IMHO, Inspection is the key: If it looks doubtful - replace it. It's a lot cheaper than the damage a not-functional air filter causes to the engine. I ran across a chap with a 280SE, where the filter had been changed by an authorized Mercedes Shop. Somehow it had been installed so that abrasive dust had ruined the cylinders. Mercedes got him a new engine. He said:"It was an uphill battle"
1969 280SL,Signal Red, A/T, P/S, A/C, Both tops. Ret. engr.