http://guitartricks.com/helptab.php#q4

The above is the GuitarTricks symbol guide for tablature. Not all tablature is written the same; some is very detailed, others are not. Some use different symbols. Usualy it's going to look something like the ones on this page. Or at least a little v pointing down for downstrokes, and a little ^ pointing up for upstrokes.

When it is not indicated then it's really a matter of timing and rhythm. A combination of up and down is common for a lot of styles.