States looking to raise revenue to fill widening budget gaps have been known to consider new kinds of tax laws. That's why wealthy clients may start hearing about “exit taxes” that would be imposed on wealthy residents who to move to another state.

State exit taxes follow the lead of the federal government, which has long imposed an exit tax on Americans who leave the U.S. Critics, however, say state-level exit taxes—California has proposed several—are cumbersome to administrate and unconstitutional under interstate commerce and due process clauses.

“California proposals have tended to include exit taxes ... that implicate a host of federal constitutional provisions,” wrote the Tax Foundation. “And most prior proposals would tax worldwide net worth for state residents, with all the constitutional questions that raises.”

Supporters counter that state exit taxes would affect only the uber-wealthy who look to avoid taxes on assets they earned while living in a state.

Many on both sides, though, think that more states may be proposing such a tax in the future as they look to generate revenue. California recently proposed such a measure as part of a wealth tax on billionaires, an attempt to recoup money on stock earnings by residents or soon-to-be ex-residents. The bill failed.

Under the California proposal, for example, a 0.4% tax would be imposed on the net worth of individuals or businesses above $30 million. The tax would be a one-time event when these taxpayers leave the state.

“California is basically looking for unrecognized capital gains for wealthy persons leaving the state,” said Mary Kay Foss, a CPA in Carlsbad, Calif. “Gains on California real estate are taxed here in any case, so the tax would be imposed either on unrecognized gains on other assets or wealth other than real estate. It doesn’t smell constitutional to me.”

Exit taxes seem to be connected to wealth taxes. Seven states—New York, California, Connecticut, Hawaii, Illinois, Maryland and Washington—have some sort of wealth tax, said Miklos Ringbauer, CPA and founder of MiklosCPA in Los Angeles.

“It’s not legal for states to implement a real exit tax on their citizens simply as a result of moving to another state,” Ringbauer said. But “states like New York, Massachusetts and California have implemented robust exit-tax audit techniques such as automated programs to track a citizen’s activities—cellphone use, the location of credit card charges, use of toll roads and so on—to help determine whether the taxpayer actually moved out of the state or has spent sufficient number of days in the state and is now subject to state income taxes.”

Other clues states can use to establish a taxpayers real domicile include state-issued drivers licenses, voter registrations or part-year or non-resident tax returns, Ringbauer added.

Exit taxes can travel under other names. New Jersey, for instance, is one state where gains on a property sale that fall outside of an exclusion are taxable for a resident who’s leaving the state. (The tax is estimated and assessed at closing.) “Taxpayers may also misclassify certain automatic tax withholdings as an exit tax,” Ringbauer said.

“Exit taxes similar to the [federal] one reported on IRS Form 1040-C most likely will not be an option [but] states will continue look for ways to track and maintain state-sourced income to be taxable even if the taxpayer moves to another state,” Ringbauer said.

More conventional taxes on the wealthy will probably be what clients must contend with. For example, Ringbauer said that stock options and certain types of deferred compensation earned while being a resident of the state are technically future taxable items when received. Another example would be selling a real property under Sec. 1031, moving to another state and repeating the process and not declaring the postponed gains after sale of the last property without a 1031 exchange.

“States may look at implementing automatic withholding requirements on more types of income and have the taxpayer file a return to claim a refund,” Ringbauer added.

Though clients may read about exit taxes and increasingly mention them, “states will probably look at wealth taxation instead of an exit tax as an alternative,” he said.