Nope, it’s an excellent question. Most tawny’s with an indication of age (10,20…) and Colheita’s now have the date bottled listed on the front or back label as David mentioned. Though not all do and it tends to be a more relatively recent thing for most. So one possible indicator, of many, the bottle was bottled a long time ago is if it doesn’t have a date listed.
Most tawny’s are not meant to be aged in bottle. Most are intended to be consumed within a few years of bottling. Some producers will age and improve in bottle, like Niepoort. However, those few producers are the exception.
This does not mean a 20 yr tawny bottled in 2000 is going to be bad, it will just be different and not what the winemaker intended the experience for the drinker to be. I’ve had old bottled tawny’s (what would be called a Reserve Tawny today, so not expensive) almost 100 years old in bottle that were still pleasant and enjoyable, not great mind you but still enjoyable.
When most tawny’s age in bottle for longer periods they tend to lose their perceived acidity and become what is best described as “flat” or cloying. That crisp brightness tends to fade away. Some people enjoy that. I prefer that crisp bright vibrant acidity which balances out the sweetness tawny’s can get from aging, especially older ones.
For professional reviewers, I know of none who actively record every bottling date for tawny’s and Colheita’s they taste and publish. So one does need to pay attention to when that review was actually written…nothing like a TN on it from 20 years ago. 
You’re best bet for that is wine forums like here or FTLOP or TPF, where us Nerds list bottling dates of what we drink.
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