You may be able to diagnose the cause of unboundedness by examining the output from the optimizer that detected the unboundedness. For example, if the presolve step at the beginning of optimization made a series of reductions and then stopped with a message like this:

Primal unbounded due to dual bounds, variable 'x1'.

it makes sense to look at your formulation, paying particular attention to variable x1 and its interactions. Perhaps x1 never intersects less-than-or-equal-to constraints with a positive coefficient (or, greater-than-or-equal-to constraints with a negative coefficient), and by inspection you can see that nothing prevents x1 from going to infinity.

Similarly, the primal simplex optimizer may terminate with a message like this:

Diverging variable = x2

In such a case, you should focus attention on x2. (The dual simplex and barrier optmizers work differently than primal; they do not detect unboundedness in this way.) Unfortunately, the variable which is reported in one of these ways may or may not be a direct cause of the unboundedness, because of the many algebraic manipulations performed by the optimizer along the way.