Regarding the third point of Anonymous's suggestion, here's an article Topical Bias in Generalist Mathematics Journals by Joseph F. Grcar in the december 2010 issue of the Notices of the American Mathematical Society.
In many branches of mathematics, the so-called "generalist" journals are perceived more prestigious and publish many of the best results. Typically they claim that they publish papers of the highest quality in all branches of mathematics. However, they don't necessarily accept papers from each field equally likely. In fact, as the linked article's figures clearly suggest, there are clear biases for/against certain branches if you count the papers of a certain topic published in generalist journals and compare the number with the total number of papers of the same kind.
Since good generalist journals are among the most prestigious publication venues in many branches of mathematics, by looking at statistics like this, you may be able to detect prejudices about branches among mathematicians, which is mentioned in the linked article. So, if your definition of "trendy" is highly correlated with this type of prejudice (which I think it is because, all else being equal, the number of publications in prestigious journals in your CV increases your chance of getting a job in academia), it can be a fairly reliable measure of trendiness.
The Annals of Mathematics Anonymous mentioned is definitely among the very top generalist journals. So checking the latest issues may tell you something about what's hot in mathematics now.
The big caveat, however, is that it only publishes 50 or so articles in each year. So, if you only look up in recent issues, your statistics can't be extremely reliable. Using a record of several years defies the purpose of knowing what's trendy right now, so you're in a bit of a conundrum. Making things more complicated, each generalist journal has their own bias as well; different journals tend to publish papers of different topics more frequently.
So, it might be better to take samples from multiple generalist journals which are considered among the top journals by many. Those journals typically publish an extremely small number of papers though. For example, the Journal of the American Mathematical Society published only 32 papers in 2011 (and 35 in 2010) according to ISI Web of Knowledge. Some prestigious generalist journals publish even fewer papers. (I'm not going to talk about which journals you should use for this purpose because it's inevitably controversial and subjective.)
In any case, if you want to check what kind of topic is discussed in a given journal, you can do that quite easily by MathSciNet. The American Mathematical Society classified branches of mathematics and gave a code number to each branch. And they've been keeping track of publications in mathematics. Just search for the MSC code of your field with a specific journal in MathSciNet and see how many hits it returns. This way, you can see how trendy your field is as well as which prestigious generalist journal likes your kind of mathematics more.
This method doesn't always work equally well though, e.g., if you're interested in applied mathematics or mathematical sides of computer science, what the Annals of Mathematics loves nowadays may not be the most reliable measure that reflects what's trendy right now. Another example is when your field became fashionable relatively recently so there isn't a good catch-all MSC code yet. For instance, arithmetic combinatorics has seen a miraculous revival in recent years in mathematics. Ann. of Math. and other prestigious journals are publishing papers in this field, but there isn't a good MSC code to capture this.