elenasgrumpy,
When I got my first 4-stroke I thought I could tell too. A buddy loaned me his tach and I realized that I didn't have a clue. So I invested in a tach. It really helps. Once you have some time with the engine you can start telling by ear what is good and what isn't, but use the tach until you get your ear trained. And besides, tach's are a good thing to have at the flying field. I don't think a weekend goes by that I'm not using my tach for something or other, especially when helping to troubleshoot engine problems for others. They're a great investment.

This is the one that I have and I really like it. It's $45.95, but it's also got an extended scale voltmeter in it that you can use to test your transmitter and receiver batteries with. I'm really happy with mine.
[link=http://www.horizonhobby.com/Shop/ByCategory/Product/Default.aspx?ProdID=HAN111]Hanger 9 tachometer/voltmeter[/link]

Hope this helps

Ken