Unfortunately there is no simple or surefire way to assess if a disc burner is getting long in the tooth and should be replaced. Yes, the drive lens may be fogged or dirty, depending on the condition of its surroundings, and a lens cleaning disc may help.
My 'indicator' that it's getting time to replace a burner is an increase in the number of failed verifications from a few % to over 10%, with the same brand of blank (Verbatim, made in Taiwan or Singapore, NOT in Indonesia or India, etc.)
In general you should know that a consumer quality burner is only good for a few hundred burns, rarely more than 300-400 (if that many). Using a burner in dusty or smoky surroundings drastically cuts into its life expectancy by fogging the drive lens, sometimes irreversibly. Cleaning discs are OK, but more effective lens cleaning is only possible when the lens is directly accessible, a feature few drives offer without disassembly.
As to recommending drive brands, I've long used LaCie, but switched to OWC because of LaCie's perennial power supply issues (the drives themselves are fine).
PS, I concur with joemikeb and would opt for an external replacement burner. I would use the extra cost of replacing the internal drive for either an (extra) external drive or for a FW hub, in case you're out of FW ports in your current configuration.
