2. Check the oil pressure. The oil pressure should be within the manufacturer's specifications. If the oil pressure is too low, then the oil cooler may not be working properly.
3. Listen for unusual noises. If you hear any unusual noises coming from the oil cooler, then it may be damaged and will need to be replaced.
4. Check the oil temperature. The oil temperature should be within the manufacturer's specifications. If the oil temperature is too high, then the oil cooler may not be working properly.
If you suspect that the oil cooler on your 99 VW Beetle is damaged, then you should take it to a mechanic for further testing and diagnosis.