Amplifying on what Grimmy said, borrow the meat thermometer from the kitchen and stick it in the roots of your bonsai - you want this temperature to stay below 90F! Standard tricks to accomplish this (if it is a problem) amount to making a swamp cooler - damp sphagnum on the soil surface or wrapping a light colored damp towel over the pot and soil.
Browning leaf margins are sympotomatic of transpiration occurring faster than the roots can take up water. If brown margins occur only on certain exposed leafs you can suspect too much wind and/or wind-heat.
Generally browning margins, on the other hand, almost certainly indicate root-related problems. Anytime your daytime termperature are above 90F, think 'cooking roots - must cool roots'. Get the pot out of direct sunlight and make a swamp cooler for your tree's roots. Stick a meat thermometer in the roots and verify the temperature.
If none of this applies or works, then suspect fungus. For the most part bugs eat tissue (i.e., parts of your leafs are missing) - exceptions being easily identified scale and aphids. Both are found on the underside of leafs and rarely, if ever, cause leaf margins to brown. Instead, they usually cause the leafs to curl more than normal.
